fix(app): address code audit findings — app module

[leszek3737]

• Fix wrong MIME types: Julia .jl → text/x-julia, bat/cmd → text/x-msdos-batch, .mm → text/x-objective-c++
• Fix case-sensitive FS mismatch in CONFIG_EXACT_NAMES (exact match on Linux, case-insensitive on macOS/Windows)
• Fix thread leak in JobRunner Drop (single reaper thread, 5s deadline)
• Fix shell error merging (log raw_err, return PermissionDenied over NotFound)
• Fix FileEntryBuilder accepting empty names
• Fix FileSize rounding edge case (while-loop for cascading promotion)
• Cache canonical path per watcher poll, dedup events per path
• Cache grapheme_count in TextInput (O(n) → O(1) cursor ops)
• Box large DialogKind variants (240B → 24B stack size)
• Bound channel in job_runner (sync_channel(128))
• Add panel history limit (256 entries)
• Add hotlist validation (filter empty/whitespace paths)
• Add BufWriter in debug_log, single stderr lock
• Add OnceLock cache for get_shell
• Add #[must_use] on paths.rs public functions
• Add SortMode methods and Hash derive
• Extract helpers in user_menu (non_utf8_err, collect_body_lines)
• Split poll_watcher_events into drain/accumulate/apply
• Improve tests: panel_with_n helper, WatcherHarness, let/else patterns, remove duplicates

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[gemini-code-assist]

Code Review

This pull request introduces various performance optimizations, refactorings, and bug fixes across the codebase, notably caching grapheme counts in TextInput, boxing large DialogKind variants to reduce stack size, caching shell paths, and batching file watcher events to reduce UI thread overhead. Feedback focuses on addressing a potential panic in ends_with_ignore_ascii_case when slicing non-char boundaries, ensuring BufWriter flushes on every write to prevent lost debug logs, removing a premature atomic optimization in directory creation, and eliminating redundant blocking disk I/O and system calls in directory tree traversal and watcher event processing.

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[greptile-apps]

Greptile Summary

This PR addresses a broad code audit across the app module, introducing ~35 targeted fixes and improvements spanning correctness bugs, resource management, and performance micro-optimisations.

• Watcher refactor (watcher_sync.rs): splits poll_watcher_events into drain_events / accumulate_changes / apply_panel_changes, caches canonical paths per poll to avoid repeated canonicalize() syscalls, and deduplicates file events by path via a HashMap. A logic bug is present in the Renamed handler where to matching either panel incorrectly schedules a full refresh on both panels.
• TextInput O(1) cursor (text_input.rs): caches grapheme_count as a struct field updated incrementally on every mutation; all existing call sites that do direct .text = assignment follow the documented cursor_end() / recompute_grapheme_count() protocol, but the pub text field leaves the invariant unenforced by the type system.
• Resource fixes: job-runner teardown drops from 3 threads to 1 polling reaper, job channel bounded at 128, shell detection cached in OnceLock, debug log wrapped in BufWriter with periodic flushing.

Confidence Score: 4/5

Safe to merge with the watcher rename-event fix applied; all other changes are well-scoped and follow existing patterns.

The watcher refactor is the highest-risk area. The Renamed event handler incorrectly fans out a DirEvent::Touch to both panels whenever either panel matches the rename target, causing the uninvolved panel to perform an unnecessary full directory read and potentially disrupting its cursor/scroll state. Every other changed file is low-risk: the TextInput cache is correct at all current call sites, the job-runner polling reaper is a clean simplification, and the boxing/MIME/config fixes are mechanical.

src/app/watcher_sync.rs — the Renamed event accumulation logic around lines 177–186

Important Files Changed

Filename	Overview
src/app/watcher_sync.rs	Significant refactor splitting poll_watcher_events into drain/accumulate/apply phases with per-poll canonical-path caching. Logic bug in Renamed handling: the `
src/app/types/text_input.rs	Adds a cached grapheme_count field to make cursor ops O(1). All current mutation sites correctly follow the update protocol; the invariant is only documented, not enforced by the type system since text remains pub.
src/app/job_runner.rs	Replaces the double-thread (reaper + join-waiter) teardown with a single polling reaper using is_finished(); bounds the job channel at 128; upgrades Ordering::Relaxed to SeqCst for cancel flag stores.
src/app/shell.rs	Caches get_shell result in OnceLock (separate caches for menu/interactive on non-Windows); removes alias wrappers; fixes the TerminalRestoreGuard double-call bug by setting already_restored unconditionally before the error chain.
src/app/types/dialogs.rs	Extracts five large DialogKind variants into separate structs and boxes them; all construction and match sites in the PR are updated correctly.
src/app/debug_log.rs	Wraps the log file in BufWriter, flushes every 256 writes rather than every write, and locks stderr once for fallback writes; also prevents redundant create_dir_all calls with DIR_CREATED flag.
src/app/mime.rs	Fixes three MIME type assignments (.jl → text/x-julia, bat/cmd → text/x-msdos-batch, .mm → text/x-objective-c++) and replaces the lowercase-then-match pattern with eq_ignore_ascii_case throughout.
src/app/types/panel.rs	Caps panel history at 256 entries; refactors sync_unfiltered_selection and source_entries; adds mark_dirty helper. Uses Vec::remove(0) for history eviction (O(n)), acceptable for 256-entry cap.
src/app/types/file_entry.rs	Guards FileEntryBuilder::build against empty names; fixes FileSize rounding via while-loop; caches display_modified in time_str to avoid repeated formatting.
src/app/config.rs	Filters empty/whitespace hotlist entries; adds debug logging for canonicalize failures; removes an unnecessary unwrap_or_default() call.
src/app/watcher_sync/tests.rs	Adds WatcherHarness and EventHarness test fixtures; replaces fragile index-based assertions with name-based lookups; adds panel_with_n helpers and let/else patterns.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[poll_watcher_events] --> B[drain_events\nup to 256 from channel]
    B --> C{events empty?}
    C -- yes --> D[return false]
    C -- no --> E[PanelCache::from_panel\nleft + right]
    E --> F[accumulate_changes\nloop over events]
    F --> G{event type}
    G -- Created/Modified --> H[event_is_panel_dir_cached\nleft? right?]
    G -- Deleted --> I[event_is_panel_dir_cached\nleft? right?]
    G -- Renamed from/to --> J[Check 'from' left/right\nCheck 'to' left/right ⚠️ OR-bug]
    G -- Overflow --> K[overflow = true]
    H --> L[AccumulatedChanges]
    I --> L
    J --> L
    K --> L
    L --> M[apply_panel_changes]
    M --> N[apply_dir_event left]
    M --> O[apply_dir_event right]
    M --> P{needs_full_refresh?}
    P -- yes --> Q[full_refresh_panels\nread_directory IO]
    P -- no --> R[apply_file_events\nper-path upsert/remove]
    Q --> S[rebuild_visible_entries]
    R --> S
    S --> T[return dirty]

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