feat(go/fio): debug logging via slog at LOG_LEVEL=DEBUG

Wires slog.SetDefault to honour LOG_LEVEL in all CLI commands and adds
debug logs on the Fio fetch path so a silent "fetched 0 transaction(s)"
can be diagnosed without code changes:

- fio.New: which client variant (api/transparent) was selected
- apiClient: GET URL (token redacted as ****), HTTP status, body bytes,
  parsed transaction count
- transparentClient: GET URL, HTTP status, body bytes, plus parser
  stats (raw rows from second table, kept, dropped_bad_date,
  dropped_nonpositive_amount)

Also suppresses the --print-fio-table block when zero transactions were
fetched, so the bare header no longer prints under that condition.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/io/fio/client.go

@@ -4,6 +4,7 @@ package fio
 
 import (
 	"context"
+	"log/slog"
 	"net/http"
 	"time"
 )
@@ -36,7 +37,9 @@ func New(token, accountNum string, hc httpDoer) Client {
 		hc = http.DefaultClient
 	}
 	if token != "" {
+		slog.Debug("fio: client selected", "type", "api")
 		return &apiClient{token: token, hc: hc}
 	}
+	slog.Debug("fio: client selected", "type", "transparent", "account_num", accountNum)
 	return &transparentClient{accountNum: accountNum, hc: hc}
 }