feat(session): bi-directional cursor-based pagination (#6548)

[CasualDeveloper]

Summary

This PR implements bi-directional cursor-based pagination for message loading to support long session histories without performance degradation. Closes #6548.

• Backend: before/after/oldest cursor support with RFC 5005 Link headers
• TUI: Infinite scroll with memory bounding (500 messages), Home/End jump navigation
• Tests: 18 tests across 3 test files

Context

Supersedes #6656 by @ry2009 (earlier partial implementation). This PR is a ground-up rewrite that adds:

• Bi-directional pagination (both before and after cursors)
• oldest param for jumping to beginning of history
• Proper parseLinkHeader utility instead of brittle string matching
• Memory bounding to prevent unbounded growth
• Error state feedback instead of silent failures
• RFC 5005 compliant Link header semantics

Changes

Backend (session/index.ts, server/server.ts, message-v2.ts)

• Add before, after, and oldest cursor params to Session.messages()
• Add ascending option to MessageV2.stream() for oldest-first iteration
• Add Binary.lowerBound to util for efficient cursor lookup
• Return 400 when mutually exclusive cursors specified
• RFC 5005 Link headers: rel="prev" → older, rel="next" → newer

TUI (sync.tsx, routes/session/index.tsx)

• message_page state tracking (hasOlder, hasNewer, loading, error)
• loadOlder/loadNewer with scroll position restoration
• jumpToOldest (Home key) / jumpToLatest (End key) for absolute navigation
• Revert-aware: jumpToLatest preserves revert marker when jumping to newest
• 500-message sliding window with 50-message eviction chunks
• Skip SSE inserts when detached from head (hasNewer=true)
• Race condition prevention: check page state before acquiring loading guard

Tests

File	Tests	Coverage
messages-pagination.test.ts	5	Cursor, boundary, deletion
session-messages.test.ts	6	API validation, Link headers, oldest param
parse-link-header.test.ts	9	RFC 8288 parsing edge cases

Testing

bun test test/session/messages-pagination.test.ts test/server/session-messages.test.ts test/util/parse-link-header.test.ts
# 18 pass, 0 fail

Related

• Closes #6548
• Supersedes #6656
• Related: #6137, #6138 (message_limit config — this makes that less necessary)