fix(tui): bound @-completion file-index walk with a wall-clock budget (#4365)

[LeoLin990405]

Problem

Fixes #4365 — @-mentioning a large non-workspace directory freezes the TUI (reporter saw an unresponsive terminal on pwsh7). The mention completion pulls the full subtree eagerly.

Root cause

The fuzzy-completion index (Workspace::build_file_index in working_set.rs) is rebuilt synchronously on the input thread whenever the walk root changes — which is every keystroke while typing an @-path outside the workspace. FILE_INDEX_MAX_ENTRIES = 50_000 caps the result size but not the time to reach it: a large external tree with no .gitignore can spend seconds walking before it hits 50k entries, and that whole time the UI is blocked.

Fix

Add a 150ms wall-clock budget (COMPLETION_WALK_BUDGET) checked alongside the existing entry cap in every walk loop of build_file_index. Once the budget is exceeded the build returns the partial index instead of continuing — partial completions are far better than an unresponsive terminal, and the next keystroke re-walks (from cache when the root is unchanged). The dot-dir and local_reference_paths passes are skipped once over budget so they can't reintroduce the freeze.

build_file_index is split into build_file_index_with_budget(budget) so the behavior is unit-testable; the public path calls it with COMPLETION_WALK_BUDGET, so nothing else changes for normal-sized workspaces (the budget is a no-op there, same as the entry cap).

Testing

• New build_file_index_respects_time_budget: a generous budget indexes every file; a zero budget stops the walk early (returns fewer entries) — proving the build can't block. Deterministic, no reliance on a real slow filesystem.
• cargo clippy -p codewhale-tui --all-features clean; cargo test -p codewhale-tui --all-features working_set green (44 passed).

Note: I couldn't reproduce the actual freeze on macOS (the report is pwsh7/Windows on a huge external tree), but the time budget is platform-agnostic hardening of the exact synchronous walk named in the issue. Happy to adjust the 150ms value or the approach (e.g. debounce until the path is committed, per the reporter's suggestion) if you'd prefer.

[github-actions]

Thanks @LeoLin990405 for taking the time to contribute.

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[Hmbown]

Thank you for taking on #4365. I cannot merge this version yet because the wall-clock check happens inside each for body, after the iterator has already completed next(). A single slow filesystem read can therefore still block the input thread past the budget. local_reference_paths(...) is also materialized before its loop can check elapsed time, and a partial cache miss may repeat the synchronous walk on later input.

Please move completion discovery off the UI/input thread (with serialized scans and stale-result protection), or make the traversal itself genuinely interruptible before a blocking next(). The regression should prove that key handling stays responsive while discovery is stalled, in addition to checking partial results. The current Lint and Windows jobs are also red. I am happy to re-review a focused update, and your contribution will retain credit when the complete fix lands.