Instead of walk-everything-first then index, workers now receive files
the instant os.walk yields them. The thread pool is open before the
walk starts; each discovered file is submitted immediately. Completed
futures are drained after each directory to keep memory flat.
Progress message shows:
"Discovering & indexing (8w): 1,234 — 5,678 found so far"
then once walk finishes:
"Indexing (8w): 8,000 / 9,100"
UI: merged Discovery + Indexing into a single "Discover + Index" phase pill.
Indeterminate progress bar stays on until total file count is known.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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