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/dashboard_default.snap

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+Dashboard
+ Entity Counts                           Projects                                Recent Activity
+────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
+ Issues: 15 open / 42                    ● 5m ago infra/platform                 No recent activity
+ MRs: 7 open / 28                        ● 12m ago web/frontend
+ Discussions: 120                        ● 8m ago api/backend
+ Notes: 350 (18% system)                 ● 4m ago tools/scripts
+ Documents: 85
+ Embeddings: 200                         Last sync: succeeded
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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