fix(macapp): restore cursor-aware Up and Down prompt-history navigation

[dennisonbertram]

Parent: #991

Summary

The completed #927 acceptance requires Up/Down prompt-history recall at the first/last composer line. Production code stores history but exposes only “recall the last prompt,” has no history cursor/forward traversal, and never calls that method from the composer.

Evidence

• macapp/Sources/GoCodeUI/RunSession.swift:25-26,74-75 — history is appended.
• RunSession.swift:205-208 — recall always selects only promptHistory.last.
• macapp/Sources/GoCodeUI/ChatView.swift:888-893 — composer handles submit and draft changes, but no Up/Down key navigation.
• Repository search finds no production caller of recallPreviousPrompt().
• feat(macapp): composer — submit, multiline, prompt history #927 is closed as completed despite this acceptance item being unreachable.

Scope

• Add a history cursor with previous/next traversal and restoration of the in-progress draft.
• Intercept Up only at the first visual line/caret boundary; intercept Down only at the last.
• Preserve normal arrow-key caret movement everywhere else.
• Reset cursor appropriately after edit/submit while keeping draft text across section navigation.
• Keep Enter/Shift+Enter behavior intact.

Tests first

• Pure history-state tests: empty, one, many, repeated previous, forward traversal, return to draft, submit reset.
• Editor-boundary tests for first/middle/last line so caret movement is not stolen.
• Production-reachability test proving composer key events invoke history navigation.
• Manual keyboard smoke with multi-line drafts.

Acceptance criteria

• Up/Down traverse prompt history only at the documented caret boundaries.
• Down past newest restores the user's pre-navigation draft.
• Normal multi-line caret movement remains native.
• Draft survives navigating away/back.
• Completed feat(macapp): composer — submit, multiline, prompt history #927 acceptance is covered by behavior tests.
• swift test, Swift build, formatting, strict format lint, and keyboard smoke pass.

[dennisonbertram]

Repair scope for PR #1021 after independent review:

• Add a failing regression for adjacent identical history entries where cursor movement returns the same draft value and SwiftUI emits no onChange.
• Repair recall bookkeeping so the next manual edit resets navigation and Down cannot clobber it.
• Preserve the documented macOS 14 draft-state approximation.

Literal caret-position support and the installed-app half-typed-draft smoke remain pending as already documented.

[dennisonbertram]

PR #1021 repair is pushed at 575b670938821f9db8b5bb096e3956753b0f0e25. #998 keeps identical prompts as distinct entries, declines no-op/oldest recalls, and correctly disarms recall bookkeeping when SwiftUI emits no value change. Prompt-history and reachability coverage pass in the full 303-test Swift run. Literal caret binding remains the documented macOS-14-floor deferral in #1020.