feat(core): deterministic session log replay with synced watermark - #35040

Summary

Replay durable Session logs through a fixed pre-captured watermark and emit log.synced exactly once, replacing the old log.caught_up moving-boundary marker. This makes reconnect-safe log replay deterministic while preserving live tail delivery for events committed during replay.

Changes

• packages/schema: rename EventLog.CaughtUp / log.caught_up to EventLog.Synced / log.synced, update marker docs, and cover optional seq encoding.
• packages/core: add bounded readAfter { through, limit }, LayerOptions.logReadPageSize defaulting to 512, pull-driven paged replay, once-only synced marker, and isCaughtUp -> isSynced.
• packages/protocol: update the Session log SSE union and route description to the log.synced contract.
• packages/client and packages/sdk-next: update marker-related tests without regenerating generated client trees.
• specs/v2: document the marker rename, deterministic watermark replay, and generated-client integration timing.

Decisions

• D4: keep aggregateID on log.synced for consistency with envelopes, hints, and core aggregate APIs.
• D5: omit seq when the captured watermark is empty (-1).
• D5: emit the marker once for the captured watermark.
• D5: expose LayerOptions.logReadPageSize, default 512.
• D5: defer session.get watermark and generalized aggregate log API.

Closes #35015
Part of #35021

Test Plan

• packages/schema: bun typecheck passed.
• packages/core: bun typecheck passed.
• packages/core: bun run test -- test/event.test.ts test/session-log.test.ts test/session-create.test.ts test/session-prompt.test.ts passed.
• packages/protocol: bun typecheck passed.
• packages/client: bun typecheck passed.
• packages/client: bun run test -- test/effect.test.ts test/promise.test.ts passed.
• packages/sdk-next: bun typecheck passed.
• packages/sdk-next: bun run test -- test/embedded.test.ts passed.
• Worktree root: bunx prettier --check packages/client/test/effect.test.ts packages/client/test/promise.test.ts packages/core/src/event.ts packages/core/src/session.ts packages/core/test/event.test.ts packages/core/test/session-create.test.ts packages/core/test/session-log.test.ts packages/core/test/session-prompt.test.ts packages/protocol/src/groups/session.ts packages/schema/src/event-log.ts packages/schema/src/event.ts packages/schema/test/event.test.ts packages/sdk-next/test/embedded.test.ts specs/v2/schema-changelog.md specs/v2/session.md passed.
• Push hook: bun turbo typecheck passed.

Generated client regeneration intentionally happens at integration time per the audit merge protocol; generated trees are untouched on this branch.