test(llm9p): add tests for per-session compact and usage files

- session_compact_test.go (llm): Tests for Session.EstimatedContextTokens,
  SessionManager.Compact (not found, too short, replaces messages, resets
  tokens), SessionManager.ContextLimit and EstimatedContextTokens.

- session_compact.go (llmfs): Fix Stat().Length for SessionCompactFile
  (was 0 from BaseFile default; now returns fixed read-msg length).

- session_compact_test.go (llmfs): Tests for SessionCompactFile (read,
  read EOF, write no-op short history, write compacts, stat), for
  SessionUsageFile (format, EOF, read-only write, stat, dynamic content),
  and SessionDir.Children/Lookup wiring for compact and usage.

52 tests total, all pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>