refactor(uninstall): split scan_applications into phase helpers

The 529-line scan_applications was carrying four logical phases plus
its own temp file, spinner subprocess, and INT trap setup. Carve the
phases into private helpers and keep scan_applications as the
orchestrator that owns the shared temp files, the INT trap (still using
`exit 130` so SIGINT terminates the script regardless of which helper is
running), the metadata cache lock context, and the empty-result
shortcuts between phases.

Helpers (underscore-prefixed; they read and write orchestrator locals
via bash dynamic scoping, so do not call them outside scan_applications):
- _scan_discover_apps: Pass 1 discovery combining the configured app
  search directories with pkg-receipt non-standard install locations,
  writing one <path|name|mtime> row per candidate to discovered_file.
- _scan_partition_cache: classifies discovered apps into warm rows
  (written directly to scan_raw_file via the nested
  use_cached_scan_metadata helper) and cold rows (queued in
  app_data_tuples for Pass 2).
- _scan_resolve_uncached: Pass 2 parallel resolve. Owns the spinner
  subprocess (spinner_pid is assigned here), fans out workers up to
  max_parallel, and waits for completion. process_app_metadata is
  defined as a nested function so worker subshells inherit it.
- _scan_finalize_index: merges scan_raw_file with the metadata cache,
  runs either the awk pipeline or the bash fallback that performs
  inline metadata fetches, persists the cache snapshot under a lock,
  sorts the result by epoch, kicks off the deferred background
  refresh, and echoes the sorted index path on success.

Behavior is preserved end to end. The two empty-result early-returns
(after partition, after Pass 2) stay in the orchestrator so the cleanup
sequence around them is unchanged.

Tests: same suite as the batch_uninstall refactor commit (uninstall,
uninstall_safety, uninstall_naming_variants, uninstall_remove_file_list,
uninstall_scan_bash32, brew_uninstall) - 95/95 pass.