refactor(core): extract transcript projection

[HOYALIM]

Issue for this PR

Closes #36809
Related to #36473

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Extracts the database-backed transcript persistence adapter from the V2 session projector into session/projection/transcript.ts, the first reviewable slice requested in #36473.

The change is mechanical: SQL queries, durable transaction ownership, projector registration order, and message update behavior remain unchanged. projector.ts now delegates transcript events and promoted-message insertion to the focused module.

How did you verify your code works?

• bun test test/session-projector.test.ts --timeout 30000 (13 passed)
• bun test --timeout 30000 --only-failures from packages/core (1,277 passed, 6 skipped, 0 failed)
• bun typecheck from packages/core
• bun lint packages/core/src/session/projection/transcript.ts (0 warnings, 0 errors)
• bunx prettier --check packages/core/src/session/projector.ts packages/core/src/session/projection/transcript.ts
• Repository pre-push bun turbo typecheck --concurrency=3 (32 successful)
• git diff --check
• Synthetic merge with current origin/v2

Screenshots / recordings

N/A; internal refactor with no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

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