Add per-destination event filtering

[ctuncay]

Summary

Adds an optional per-destination filter block to the emulator config:

destinations:
  - id: my-projection
    description: ...
    sources: [my-source]
    endpoint: http://backend:8080/api/v1/context/projection/my-projection
    filter:
      column: event_type
      values: ["OrderCreated", "OrderUpdated"]

Records whose column value is not in values are skipped and committed instead of delivered. Destinations without a filter are untouched.

Motivation: consuming apps register many HTTP destinations over the same topic, and every destination receives every record just to discard the irrelevant ones. On Sumori's local stack (34 destinations), ~91% of order-event deliveries are receive-and-discard waste. Filtering at the emulator removes that fan-out; the consumer's own accept() decoding remains the correctness gate — the filter is purely an optimization.

Fail-open semantics: records missing the filter column, or whose value is not a string, are delivered. A config mistake degrades to the pre-filter behaviour, never to silent data loss. Empty values lists are rejected at config parse time.

Compatibility — safe to release and roll out

• Backward compatible (new emulator, old configs): filter is optional. Any existing config runs unchanged with identical delivery behaviour — the filtering code path is only reached when a destination declares a filter. Projects that don't use filtering are unaffected by upgrading the image.
• Forward compatible (old emulator, new configs): current released versions parse destination objects leniently and ignore unknown keys, so a config that already contains filter blocks still loads on an old image — it just delivers everything, which is the pre-filter behaviour. Config rollout and image rollout can therefore happen in either order.
• Consumer-group cursor/state format is untouched; a filtered record advances the cursor exactly like a delivered one, so upgrading or rolling back the image mid-stream is safe.

Testing

• Unit: 6 projector specs (match / skip / fail-open on missing column and non-string values), 3 config parse specs (incl. empty-values rejection).
• E2E: file-source test proving a filtered record is committed and does not block its partition.
• Full suite locally on macOS: 286 examples, 0 failures, including the PostgreSQL, MySQL and SQL Server connector suites (recipe documented in the new CLAUDE.md).

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