fix(core): reuse durable event codecs

[opencode-agent]

Summary

• cache durable-event schema encoders by immutable event definition
• cache matching decoders at replay and synchronization boundaries
• keep serialized event shapes and validation behavior unchanged

Verification

• bun test test/session-tool-progress.test.ts from packages/core (1 passed)
• git diff --check

Requested by: @rekram1-node (Aiden via Slack)

[Enough1122]

AI code review — automated review for reference; please use your judgment.

• packages/core/src/bus.ts:70 — The WeakMap fast path only pays off if Event.Definition instances are stable singletons; if Durable.get ever reconstructs or wraps definitions per lookup, every call becomes a miss plus an extra map write (a small regression instead of a win) — worth asserting definition identity stability in the durable registry or adding a debug counter to confirm hit rate.
• packages/core/src/bus.ts:79 — Cached decoders silently pin the schema captured at first use: if a definition's data schema is mutated or its event type is re-registered at runtime (dynamic registration, dev hot-reload), events keep validating against the stale codec with no error — either document that definitions must be immutable, or clear these caches wherever the Durable registry is updated.
• packages/core/src/bus.ts:282 — Both call sites still duplicate the as Record<string, unknown> cast after the helper returns unknown — moving the cast inside encodeData (typed return) would remove the repetition and give callers a safer contract.
• packages/core/src/bus.ts:70 — No regression test accompanies the change; a cheap guard would round-trip the same durable event twice and spy that the second encode/decode reuses the cached codec (or simply assert identical output across many iterations), so future refactors of the cache don't silently change validation semantics.

— AI code review (automated)

[github-actions]

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