Add a log_level build option and teardown tracing

[lukashes]

What

• New -Dlog_level build option (err, warn, info, debug; default info). Raises the compiled-in log level so std.log.debug is emitted; default info keeps Release/prod output clean. The value is validated at zig build time. Wired through build_options into std_options.
• The load stand Dockerfile builds with -Dlog_level=debug (keeps ReleaseFast perf, gains the traces).
• Debug traces along the shutdown path: produce-failed propagation, the flush worker cancel, and processor/producer deinit.
• Drop the two per-message debug logs from the hot loop ("Sent message", "No matching streams") so raising the level does not flood the output.

Why

std.log.debug was compiled out of the ReleaseFast load-stand build, so a hung teardown showed no detail. This makes the fail-fast path observable on the stand.

Independent of the replication work; targets main.

[github-actions]

📊 Benchmark Results

Current run is the minimum over 3 passes, compared against the base branch (main @ c98e9cc), built on the same runner.

Benchmark	Baseline	Current	Δ Time	Allocs	Status
Converter DELETE	102.88μs	104.58μs	+1.6%	12 → 12	➡️
Converter INSERT	100.45μs	101.58μs	+1.1%	12 → 12	➡️
Converter UPDATE	141.19μs	141.31μs	+0.1%	21 → 21	➡️
JsonSerializer	34.59μs	34.90μs	+0.9%	3 → 3	➡️
KafkaProducer produce	253.01μs	252.51μs	-0.2%	1 → 1	➡️
KafkaProducer sendMessage	305.34μs	311.35μs	+2.0%	0 → 0	➡️
PgOutputDecoder	77.18μs	78.88μs	+2.2%	6 → 6	➡️
getPartitionKeyValue boolean	13.61μs	13.53μs	-0.6%	1 → 1	➡️
getPartitionKeyValue integer	40.88μs	41.31μs	+1.0%	3 → 3	➡️
getPartitionKeyValue not found	0.03μs	0.03μs	+0.0%	0 → 0	⚪ noise
getPartitionKeyValue string	15.07μs	15.15μs	+0.5%	1 → 1	➡️
matchStreams found	14.97μs	15.12μs	+1.0%	1 → 1	➡️
matchStreams not found	0.14μs	0.04μs	-69.6%	0 → 0	⚪ noise

Summary: ➡️ 11 neutral · ⚪ 2 ignored (sub-μs)

_{Thresholds: <1μs ignore · 1–20μs 15% · 20–50μs 10% · ≥50μs 5%. Measured on a shared CI runner — treat small deltas as noise. Informational only; this check never fails the build.}