feat(tui): add snapshot test infrastructure + terminal compat matrix (bd-2nfs)

- 6 deterministic snapshot tests at 120x40 with FakeClock frozen at 2026-01-15T12:00:00Z
- Buffer-to-plaintext serializer resolving chars, graphemes, and wide-char continuations
- Golden file management with UPDATE_SNAPSHOTS=1 env var for regeneration
- Snapshot diff output on mismatch for easy debugging
- Tests: dashboard, issue list, issue detail, MR list, search results, empty state
- TERMINAL_COMPAT.md template for manual QA across iTerm2/tmux/Alacritty/kitty/WezTerm

crates/lore-tui/tests/snapshots/empty_state.snap

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+Dashboard
+ Entity Counts                           Projects                                Recent Activity
+────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
+ Issues: 0 open / 0                      No projects synced                      No recent activity
+ MRs: 0 open / 0
+ Discussions: 0
+ Notes: 0 (0% system)
+ Documents: 0
+ Embeddings: 0
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+ NORMAL  q:quit  esc:back  ?:help  C-p:palette  o:browser  P:scope  gh:home  gi:issues  gm:mrs  g/:search  gt:timeline