## Context

Continuing the Go backend rewrite tracked in
[2026-05-03-2349-go-backend-rewrite-progress.md](../../srv/personal/fuj-management/docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md).
M2.1–M2.5 are landed. Next leaf-level pure function is `generate_sync_id`
from [scripts/sync_fio_to_sheets.py:62-77](../../srv/personal/fuj-management/scripts/sync_fio_to_sheets.py#L62-L77).

It computes a SHA-256 hash over a fixed seven-field projection of a Fio
transaction (`date|amount|currency|sender|vs|message|bank_id`) and is
the deduplication key written into column K (`Sync ID`) of the payments
sheet. The Go port must produce a **byte-identical** digest for the same
transaction; otherwise the Go-side sync (M4.7) would re-append rows
already written by the Python sync, double-counting payments.

The non-trivial part is the `amount` field's string serialisation:
upstream `fio_utils.py` always supplies `amount` as a Python `float`
(API path: `float(val(1) or 0)`; HTML path: `parse_czech_amount(...)`
which returns `float`). Python's `str(float)` produces `"500.0"` for
whole-valued floats; Go's `strconv.FormatFloat(f, 'g', -1, 64)` produces
`"500"`. This is the gotcha called out in the M2.6 line of the progress
tracker.

## Python behaviour (the spec)

```py
def generate_sync_id(tx: dict) -> str:
    components = [
        str(tx.get("date", "")),
        str(tx.get("amount", "")),
        str(tx.get("currency", "CZK")),
        str(tx.get("sender", "")),
        str(tx.get("vs", "")),
        str(tx.get("message", "")),
        str(tx.get("bank_id", "")),
    ]
    raw_str = "|".join(components).lower()
    return hashlib.sha256(raw_str.encode("utf-8")).hexdigest()
```

Behavioural notes for the Go port:

1. **Field order is load-bearing.** `date|amount|currency|sender|vs|message|bank_id` exactly.
2. **Separator is `"|"`.**
3. **Whole string is `.lower()`-ed before hashing** (so e.g. "ABC" sender vs "abc" hash identically). Unicode lower; in practice Fio data is ASCII + Czech diacritics.
4. **`currency` defaults to `"CZK"`** when missing from the dict (HTML scraper path never sets it). Other fields default to `""`.
5. **`amount` is a `float`.** Always. Real Fio data is `500.0`, `1234.56`, etc. — no NaN/Inf, but parity test must pin the format.
6. **Output is `hashlib.sha256(...).hexdigest()`** — 64-char lowercase hex.
7. **Encoding is UTF-8.**

### `str(float)` cases observed in real Fio amounts

| float64 | Python `str(f)` | Go `strconv.FormatFloat(f,'g',-1,64)` | Need |
|---|---|---|---|
| `500.0` | `"500.0"` | `"500"` | append `.0` |
| `1234.56` | `"1234.56"` | `"1234.56"` | matches |
| `0.0` | `"0.0"` | `"0"` | append `.0` |
| `-500.0` | `"-500.0"` | `"-500"` | append `.0` |
| `0.1` | `"0.1"` | `"0.1"` | matches |
| `99999.99` | `"99999.99"` | `"99999.99"` | matches |

For the Fio amount domain (signed CZK, ≤ ~7 digits, ≤2 decimal places),
the rule "`'g'` with prec -1, then append `.0` if result has no `.` and
no `e`/`E`" is exact. We do not need to handle Python's
scientific-notation crossover (`>= 1e16`) for real data, but the
implementation should still cope with it correctly via the same rule.

## Approach

Create new package `internal/domain/synch` mirroring the layout of
`internal/domain/money` (single-file module + test file alongside).

### Package + signature

```go
// Package synch ports the bank-sync deduplication helper from
// scripts/sync_fio_to_sheets.py.
package synch

// Transaction is the projection of a Fio transaction that participates
// in the Sync ID hash. Other fields (ks, ss, sender_account, …) are
// intentionally excluded — they are not part of the Python hash.
//
// Currency: leave "" to inherit the Python default of "CZK" (matches
// the HTML scraper path which omits the key entirely).
type Transaction struct {
    Date     string
    Amount   float64
    Currency string
    Sender   string
    VS       string
    Message  string
    BankID   string
}

// GenerateSyncID returns the lowercase SHA-256 hex digest of
// "date|amount|currency|sender|vs|message|bank_id" (lower-cased), used
// as the dedup key in column K of the payments sheet.
//
// Byte-stable with scripts/sync_fio_to_sheets.py generate_sync_id.
func GenerateSyncID(tx Transaction) string
```

### `Currency` default

In Go every struct field is always present, so we lose Python's
"missing key vs empty string" distinction. Real-world data either sets
`currency = "CZK"` (API path) or omits the key (HTML path → `"CZK"`
default). Empty string never occurs in practice. The Go port collapses
the two by treating `Currency == ""` as "use `CZK`":

```go
currency := tx.Currency
if currency == "" {
    currency = "CZK"
}
```

This is byte-equal to Python for every input we will ever see in
production, and avoids forcing callers to pass a `*string`.

### Float formatter

Internal helper, unexported:

```go
// formatAmount mimics Python's str(float) for the float values that
// appear in Fio transactions. For mundane decimal amounts the rule
// is: format with 'g' precision -1, then append ".0" if the result
// has no decimal point and no exponent.
func formatAmount(f float64) string {
    s := strconv.FormatFloat(f, 'g', -1, 64)
    if !strings.ContainsAny(s, ".eE") {
        s += ".0"
    }
    return s
}
```

Tested explicitly (see Tests below) so the edge cases (`0`, whole
numbers, negatives, large/small with exponent) stay locked.

### Hash composition

```go
func GenerateSyncID(tx Transaction) string {
    currency := tx.Currency
    if currency == "" {
        currency = "CZK"
    }
    raw := strings.ToLower(strings.Join([]string{
        tx.Date,
        formatAmount(tx.Amount),
        currency,
        tx.Sender,
        tx.VS,
        tx.Message,
        tx.BankID,
    }, "|"))
    sum := sha256.Sum256([]byte(raw))
    return hex.EncodeToString(sum[:])
}
```

(`crypto/sha256` + `encoding/hex` — both stdlib, no `go.mod` change.)

## Tests

`synch_test.go` mirrors `money_test.go`'s table-driven style with the
verification snippet at the top of the function. Two test functions:

### 1. `TestGenerateSyncID`

Each row's expected digest is computed from the Python source:

```sh
PYTHONPATH=scripts:. python -c '
from sync_fio_to_sheets import generate_sync_id
cases = [
    {"date":"2026-01-15","amount":500.0,"currency":"CZK","sender":"Jan Novak","vs":"123","message":"clenske 1/2026","bank_id":"abc123"},
    {"date":"2026-01-15","amount":500.0,"sender":"Jan Novak","vs":"123","message":"clenske 1/2026","bank_id":"abc123"},  # currency missing → CZK
    {"date":"2026-02-10","amount":1234.56,"currency":"CZK","sender":"ABC SRO","vs":"","message":"FAKTURA 42","bank_id":"xyz"},  # mixed case → lowercased
    {"date":"2026-03-01","amount":-500.0,"currency":"CZK","sender":"refund","vs":"","message":"","bank_id":""},  # negative
    {"date":"2026-04-01","amount":0.0,"currency":"CZK","sender":"","vs":"","message":"","bank_id":""},  # zero amount
    {},  # empty dict — every field falls back to default
]
for c in cases:
    print(repr(c), "->", generate_sync_id(c))
'
```

Cases (one row per dict above), each asserting the exact 64-char hex
digest the snippet prints. Cover:

- Happy path with all fields set.
- `Currency: ""` → `"CZK"` default (parity with missing key).
- Mixed-case sender/message → lowercased before hashing.
- Negative amount.
- Zero amount.
- Zero-value `Transaction{}` — every field at Go zero, currency defaults
  to `"CZK"`, hash matches Python `generate_sync_id({})`.

### 2. `TestFormatAmount`

Pin the float formatter against Python's `str(float)`:

```sh
PYTHONPATH=scripts:. python -c '
for v in [0.0, 500.0, -500.0, 0.1, 1234.56, 99999.99, 1500000.0, 1e16, 1e-5]:
    print(repr(v), "->", repr(str(v)))
'
```

Table of `(float64, expected string)` pairs. Whole numbers must end in
`.0`; existing decimal representations pass through unchanged;
exponent-form floats (`1e16`, `1e-5`) keep their format.

## Files to create

- `go/internal/domain/synch/synch.go` — package, `Transaction`,
  `GenerateSyncID`, internal `formatAmount`.
- `go/internal/domain/synch/synch_test.go` — `TestGenerateSyncID` +
  `TestFormatAmount`.

No existing Go files need editing.

## Verification

```sh
cd go && go test ./internal/domain/synch/...
make go-lint
make go-build   # sanity: nothing else broke
```

Plus run the two Python snippets in the Tests section and diff their
output against the test tables to confirm parity.

## Out of scope (explicit non-goals)

- **Hooking into the Tier-1 parity runner.** That comes with M3.5
  (`-tags=parity` build constraint and `tests/fixtures/pure/`). M2.6
  ships with hand-written, Python-verified test tables — same approach
  used by M2.1–M2.5.
- **A richer `Transaction` struct** covering ks/ss/note/sender_account.
  Those fields aren't part of the hash. M4.4 (Fio IO adapter) will
  decide whether to reuse `synch.Transaction` or define its own struct
  and convert at the boundary.
- **Polymorphic input** (e.g. accepting a `map[string]any`). Python's
  duck-typing is a non-goal in Go.
- **Any Python callsite migration.** `sync_fio_to_sheets.py` keeps using
  its own `generate_sync_id` until M4.7 ports the sync service.

## Progress tracker + changelog

After the commit lands:

- Tick `M2.6` in
  [docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md](../../srv/personal/fuj-management/docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md)
  with the commit SHA, mirroring the M2.5 entry style.
- Add a `CHANGELOG.md` entry at top:
  `## YYYY-MM-DD HH:MM TZ — feat(go/M2.6): port domain/synch.GenerateSyncID`.

Branch: `feat/m2-6-synch-generate-sync-id` (per CLAUDE.md
branch-per-feature workflow). Push, open MR via `tea pr create`, leave
merge to the user.