Validate REPLICA IDENTITY FULL for delete-tracking streams at startup

[lukashes]

Closes #79.

Problem

The docs require REPLICA IDENTITY FULL, but nothing checked it. With the default identity the pipeline runs fine, and only DELETE events silently drop the non-key columns from the old row, contradicting the documented format. The InvalidReplicaIdentity error was declared in the validator but never used.

Solution

• Add PostgresValidator.checkReplicaIdentityFull: reads pg_class.relreplident and fails with an actionable ALTER TABLE ... REPLICA IDENTITY FULL message unless the identity is f.
• Run it per stream during startup validation, but only when the stream tracks DELETE (Stream.tracksDelete). Replica identity is irrelevant for INSERT, and UPDATE emits only the new row, so an insert/update-only stream is not rejected and does not pay FULL's extra UPDATE WAL.
• Fail fast at startup rather than guarding at runtime: a wrong identity is a config/setup problem the operator must fix.

Test

• Integration: FULL table passes, a default-identity table errors with InvalidReplicaIdentity, and the check reports ConnectionFailed when not connected.
• Unit: Stream.tracksDelete is false for insert/update-only and true when delete is present, proving delete-free streams skip the check.

Docs (README, AGENTS.md) updated: FULL is needed on tables whose stream tracks DELETE, not on every tracked table.

[github-actions]

📊 Benchmark Results

Current run is the minimum over 3 passes, compared against the base branch (main @ a93ff4b), built on the same runner.

Benchmark	Baseline	Current	Δ Time	Allocs	Status
Converter DELETE	127.29μs	126.56μs	-0.6%	12 → 12	➡️
Converter INSERT	121.42μs	120.76μs	-0.5%	12 → 12	➡️
Converter UPDATE	172.77μs	172.71μs	-0.0%	21 → 21	➡️
JsonSerializer	40.93μs	41.07μs	+0.3%	3 → 3	➡️
KafkaProducer produce	299.02μs	283.01μs	-5.3%	1 → 1	🟢 faster
KafkaProducer sendMessage	345.37μs	349.44μs	+1.2%	0 → 0	➡️
PgOutputDecoder	84.22μs	93.77μs	+11.3%	6 → 6	🔴 slower
getPartitionKeyValue boolean	16.31μs	16.13μs	-1.1%	1 → 1	➡️
getPartitionKeyValue integer	50.69μs	50.31μs	-0.8%	3 → 3	➡️
getPartitionKeyValue not found	0.03μs	0.03μs	+0.0%	0 → 0	⚪ noise
getPartitionKeyValue string	18.41μs	18.27μs	-0.8%	1 → 1	➡️
matchStreams found	17.32μs	17.27μs	-0.3%	1 → 1	➡️
matchStreams not found	0.05μs	0.05μs	+0.0%	0 → 0	⚪ noise

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