v0.7.2: message search mode — navigate, highlight, jump — TDD

Search mode activated by /search <query>. State fields: :search-mode,
:search-query, :search-matches, :search-match-idx. Up/Down arrows
navigate between matches, Enter jumps to current match, Escape exits.

search-highlight wraps matching substrings in **bold** for markdown
rendering. View-chat shows search header bar with match count and
current position.

- channel-tui-state: 4 search state fields in init-state
- channel-tui-main: modified /search handler, search-mode key handlers
  (Up/Down/Enter/Escape), 3 new tests (activate, escape, nav)
- channel-tui-view: search-highlight fn, search header bar,
  highlighted content in count+render loops
- TUI Main: 97/98 (1 pre-existing flake)  View: 29/29

org/channel-tui-state.org

@@ -136,7 +136,9 @@ See *tui-theme-presets* for named presets (dark, light, solarized, gruvbox).")
               :pending-ctrl-x nil
               :scroll-at-bottom t :scroll-notify nil
               :streaming-text nil :url-buffer nil            ; v0.7.1
-              :collapsed-gates nil                         ; v0.7.2
+              :collapsed-gates nil                          ; v0.7.2
+              :search-mode nil :search-query ""           ; v0.7.2
+              :search-matches nil :search-match-idx 0
               :dirty (list nil nil nil))))
 #+end_src