feat(search): add Alt+C case-insensitive toggle

[roramirez]

Summary

Scrollback search is regex-based and was always case-sensitive. This adds an Alt+C toggle inside Search mode that flips case sensitivity and recomputes matches live. When case-insensitive matching is active, the status bar search info shows a trailing [i] indicator.

Changes

• src/search.rs: compute_search_matches now takes a case_insensitive: bool and builds the regex via regex::RegexBuilder::new(query).case_insensitive(..).build() (still falls back to empty matches on invalid regex).
• src/app_state.rs: add search_case_insensitive: bool to AppState (default false), thread it into update_search_matches.
• src/app_event.rs: handle_search_key detects Alt+C (checking self.modifiers.state().alt_key()) before the copy-current-match interception, flips the flag, and recomputes. Plain c / Ctrl+C still copy the current match.
• src/renderer/text.rs + render_ops.rs: thread the flag through draw → draw_status_bar → draw_search_info, rendering the [i] indicator.
• Tests: search_test.rs (case-sensitive vs case-insensitive match counts), app_state_test.rs (toggling the flag and recomputing changes the match count); existing renderer/scenario call sites updated for the new parameter.
• Docs: CHANGELOG.md, doc/SPEC.md, README.md.

How to test

• cargo test — new tests case_sensitive_misses_differing_case, case_insensitive_matches_all_cases, toggling_case_insensitive_changes_match_count pass; full suite green.
• Manually: enter Search mode with /, type a lowercase query that appears in mixed case, press Alt+C — match count increases and [i] appears in the status bar; press Alt+C again to revert. Confirm plain c still copies the current match.