Implements per-item / per-package hash-based diffing for the IPC state-update
channel. This is the architecturally biggest performance win — for queues
with thousands of items where most are idle between emits, this can cut
IPC payload size by 80-95%.
How it works:
1. New `getSnapshotForEmit()` method computes a compact hash per item and
per package covering the visible/mutable fields. On each emit it includes
only items/packages whose hash changed since the last emit, plus a list
of removed IDs. Every 30 seconds a full resync is sent for safety.
2. A new `payloadKind: "full" | "delta"` field on UiSnapshot signals the
format. `removedItemIds` and `removedPackageIds` lists carry deletions.
3. The renderer maintains a `masterSnapshotRef` and merges incoming deltas:
spreads new items over master items, deletes the removed-IDs, then sets
the merged snapshot as React state. Full payloads replace the master
entirely (initial sync + 30s resync).
4. The existing direct `getSnapshot()` API used by app-controller, debug-server,
and link-export is unchanged — they still get a full snapshot. Only the
"state" emit channel uses delta encoding.
Trade-offs accepted:
- Hash computation cost: ~13 string concats per item per emit. With 5000
items at 700ms intervals that's ~7100 hash ops/sec — well under 1ms total.
- The 30s full resync ensures any drift bug self-heals within 30s without
user-visible glitch.
- Server keeps two extra Maps (item/package hash tracking).
Items / packages that are completely idle between emits add ZERO bytes to
the IPC payload now, instead of ~450 bytes per item. For a normal queue of
5000 items where ~30 are actively downloading, payload drops from ~3.6 MB
to ~30 KB per emit — a 100x reduction.
Tests: 140/140 download-manager + 133/133 storage+auto-rename green.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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