Reject unknown TOML config keys

[lukashes]

Problem

zig-toml maps only keys that match a struct field and drops the rest with no
signal. A typo silently changes behavior: [observabilty] disables metrics,
routing_kye falls back to the default routing key.

Solution

• Parse the config a second time as a raw toml.Table and walk it against the
  Config field set, failing fast with UnknownConfigKey on the first key that
  maps to nothing.
• Descend into nested tables and [[streams]] arrays; the logged path names the
  offending key (source.postgres.slot_naem).
• The check runs before the typed parse in both loadFromTomlFile and
  loadFromTomlString.

Fixes #100.

[github-actions]

📊 Benchmark Results

Current run is the minimum over 3 passes, compared against the base branch (main @ 15023f9), built on the same runner.

Benchmark	Baseline	Current	Δ Time	Allocs	Status
Converter DELETE	127.29μs	127.24μs	-0.0%	14 → 14	➡️
Converter INSERT	127.42μs	126.72μs	-0.6%	14 → 14	➡️
Converter UPDATE	174.43μs	174.28μs	-0.1%	23 → 23	➡️
JsonSerializer	49.94μs	49.94μs	-0.0%	3 → 3	➡️
KafkaProducer produce	210.97μs	218.96μs	+3.8%	1 → 1	➡️
KafkaProducer send	252.06μs	244.95μs	-2.8%	0 → 0	➡️
PgOutputDecoder	86.86μs	96.66μs	+11.3%	6 → 6	🔴 slower
getPartitionKeyValue boolean	15.90μs	15.88μs	-0.1%	1 → 1	➡️
getPartitionKeyValue integer	0.04μs	0.04μs	-4.8%	0 → 0	⚪ noise
getPartitionKeyValue not found	0.03μs	0.03μs	+0.0%	0 → 0	⚪ noise
getPartitionKeyValue string	17.63μs	17.68μs	+0.3%	1 → 1	➡️
matchStreams found	17.80μs	17.70μs	-0.6%	1 → 1	➡️
matchStreams not found	0.06μs	0.06μs	+0.0%	0 → 0	⚪ noise

Summary: ➡️ 9 neutral · 🔴 1 slower · ⚪ 3 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.}