Find + Replace across editor surfaces

[Shpigford]

Summary

• Adds find + replace + Replace All to Clearly. The existing find bar gains a chevron-toggleable replace row, case-sensitive (Aa) and regex (.*) toggles, and Replace / Replace All buttons on Mac, iOS, and the live-preview editor; ClearlyCore gets a regex-capable TextMatcher and a new ReplaceEngine (literal + \$0–\$9 capture-group templates).
• iOS gains its first UndoManager-backed programmatic-edit path so Replace / Replace All produce a single undoable step that restores both text and syntax-highlight state.
• Documents four session learnings in CLAUDE.md: @Published willSet timing for Combine sinks, the SwiftUI .keyboardShortcut ⌥⌘-letter dispatch bug (with the existing NSMenuItem injection workaround), the iOS find-highlight removeAttribute(.backgroundColor) wipe requirement, and the worktree-local -derivedDataPath rule.

Fixes #250

Test plan

• ⌘F shows the single-row find bar; chevron expands the replace row.
• Type a query, click Replace → replaces current match and advances to the next at-or-after the replacement end; click Replace All → all replaced in one ⌘Z step; "Replaced N occurrences" inline label clears after ~2.5s.
• Toggle Aa and .* after typing a query → matches re-compute live; invalid regex shows a red border.
• iOS: same flows on the simulator with hardware keyboard; verify ⌘Z after Replace All restores text and highlights, and that successive keystrokes don't leave stale yellow fragments behind.
• swift test --package-path Packages/ClearlyCore — 172/172 pass (9 existing TextMatcher + 7 new ReplaceEngine).