feat: warn on cold cache miss + per-item progress for touch metrics

[stephenc222]

Closes #55

Summary

• Cold-cache warning: when 50+ functions are missing from the touch cache, prints a note before phase 2 starts with the miss count and a rough time estimate. Users know why analysis is slow and that the second run will be fast.
• Per-item progress during phase 2: progress callbacks now fire once per resolved miss as results drain from the channel (phase 3 loop), instead of jumping from (hits, total) directly to (total, total). The phase 3 drain is single-threaded so no Sync constraints.
• --per-function-touches help text: documents the cache location, the cold/warm cost tradeoff, and the 50-miss warning threshold.
• New test: test_populate_touch_metrics_progress_fires_per_miss — verifies phase 1 fires (0, 1) and phase 3 fires (1, 1) for a single-function cold-cache miss.

Test plan

• 313/313 tests pass
• hotspots analyze . --per-function-touches on a cold cache shows the note with count + estimate
• Second run produces no note (warm cache)
• Progress bar advances per function during phase 2 (not a single jump at the end)

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