fix(#805): reduce DEBUG log noise from binlog reader

[morgo]

Summary

Trims two large sources of noise from CI DEBUG logs (the bulk of the volume in #805) and replaces aggregate signal lost from the trimming with a single bookend log per watermark-optimization phase.

• pkg/repl/client.go — enumerate the five binlog stream-housekeeping events (GTIDEvent, TableMapEvent, XIDEvent, FormatDescriptionEvent, PreviousGTIDsEvent) as explicit empty cases in the type switch. They fall through silently rather than tripping the "Received unknown event type" log (~68k lines on the sample run). The default case still logs, so a future row-event variant we don't recognize will not be silently dropped.
• pkg/repl/client.go — drop the per-rotate Debug log (~4.5k lines on the sample run). The rotate handler logic is unchanged.
• pkg/repl/subscription_{map,buffered,queue}.go — add atomic counters (keysAdded, keysDroppedAbove, keysSkippedBelow) and emit a single Info-level \"watermark optimization toggled\" log on each SetWatermarkOptimization call with table, enabled, all three counters, and current delta_len. Counters are swapped to zero on the toggle so each phase reports its own totals. The existing per-row Debug logs are kept for zoom-in debugging — the bookend log is what makes them grep-skippable rather than essential.
• pkg/repl/subscription_{map,buffered}_test.go — three existing tests now assert the path taken via the new counters in addition to the post-state:
  • TestKeyAboveWatermark
  • TestFlushUnderLockBypassesWatermark
  • TestBufferedMapFlushWithoutLockRespectsWatermark

Closes #805. Companion to the diagnostic logging in #750 for #746 — makes those logs much easier to read on the next CI hit.

Test plan

• go build ./... clean
• go vet ./... clean
• go test ./pkg/repl/ -count=1 green locally against MySQL 8.0
• CI green
• Spot-check a fresh CI DEBUG log to confirm the bookend log appears at expected transitions and the volume drop is in line with the estimate (~38% from the trimming alone)

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