test: add baseline table-driven tests for listener build, merge and compare

Covers current correct behaviour of the pure functions in internal/gateway.
Known-buggy inputs (wildcards, collisions, truncation) are left out on purpose.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plans-executions/2026-10-08-2028-house-conventions.md

@@ -7,7 +7,7 @@ Plan: `docs/plans/2026-10-08-2028-house-conventions.md`
 - [x] Step 2 — `.claude/` and docs scaffolding
 - [x] Step 3 — Makefile, lint, formatting
 - [x] Step 4 — Module path and version identity
-- [ ] Step 5 — Baseline tests
+- [x] Step 5 — Baseline tests
 - [ ] Step 6 — CI build workflow
 - [ ] Step 7 — Close out
 
@@ -113,3 +113,20 @@ flag wiring is verified by `--help` output and the build only. `GOARCH` defaults
 `amd64` when `TARGETARCH` is empty (legacy builder, as on the CI runner) and follows
 the host under BuildKit, so a local build on an Apple Silicon machine is arm64.
 `docs/flows.md` still cites pre-change `main.go` line numbers in §2.
+
+## Step 5 — Baseline tests
+
+`internal/gateway/patch_test.go`: five table-driven tests over the pure functions.
+
+```
+$ go test -race -cover ./...
+ok  .../internal/gateway   1.870s  coverage: 96.9% of statements
+ok  .../internal/version   2.196s  coverage: 94.7% of statements
+```
+
+Worth noting: `internal/controller` and `main` are at 0%; reconciler tests with the
+fake client come with the first reconcile fix. Wildcard names, name collisions and
+truncation are deliberately not asserted, so the suite does not pin behaviour that
+the bug-fix phase will change. `TestListenersEqual_managedFields` only asserts the
+four fields the comparison covers today; it says nothing about protocol, allowed
+routes or TLS mode.