Scan tree, snapshots and what grew since last time

[kingchenc]

Closes out the roadmap: "what grew since last time" is implemented end to end, drill-in/drill-up no longer re-walk the disk, and a handful of smaller gaps found while reading the code are fixed.

Core (diskghost-core)

• ScanTree — one walk builds the whole directory tree in memory (arena + path index). report(path, top_n) returns the same ScanReport a fresh scan of that folder would, for the root or any sub-folder, with no further I/O. remove(path) takes a deleted file or folder out of the tree and fixes every ancestor's totals in place. scan_with_progress now runs on top of it (same output as before; measured on a 4.1 M-file / 489 k-dir tree: 68 s vs 70 s on main).
• Snapshot + diff() — a snapshot records every directory's subtree totals plus the biggest files, as versioned JSON (SNAPSHOT_FORMAT). diff(old, new, at) reports grown / shrunk / added / removed immediate sub-folders of at (root or any sub-folder), with totals and file-count deltas. SnapshotError covers I/O, malformed JSON, foreign format, root mismatch and unknown folder.
• ScanReport.cancelled — a cancelled walk is now distinguishable from a complete one (#[serde(default)], so old JSON still parses). FileEntry, DirSize, ScanReport gain Deserialize.
• Windows on-disk size — SetLastError(0) before GetCompressedFileSizeW, so a legitimate 0xFFFFFFFF low dword is no longer mistaken for a stale failure.
• Tests: tree build / drill / remove / cancel, snapshot round-trip + format errors, diff at root and sub-folder, root mismatch, cancelled duplicate search, human_delta, UTC timestamp rendering (28 unit tests, was 13).

CLI

• scan <dir> --save FILE writes a snapshot; --since FILE prints what changed since it (JSON: a diff object next to the report fields). New diff OLD NEW [--at DIR] [--top N] compares two saved snapshots. A cancelled scan prints a partial-result notice.
• Integration tests for save → since → diff (human + JSON, --at, identical snapshots) and for every error path (8 tests, was 6).

GUI

• Backend keeps the ScanTree of the last complete walk (with the options it was walked with); any folder below its root is answered from memory, refresh=true forces a walk. A clean delete updates the tree in place; a failed one or a real reclaim drops the cache. Every complete walk is stored as a snapshot under the app data dir (one history per root, 20 kept); list_snapshots / diff_since expose it. Cancelled walks are neither cached nor recorded.
• Frontend: Since <date> picker + Show changes → total-delta stat, ± badge per folder row, treemap tiles tinted by growth/shrinkage (new folders dashed), drillable grown/shrunk/new/removed lists. Rescan button. Cancelled scans show a notice and a partial-result stat. Breadcrumb for a drive root now scans C:\ (was C:, which Windows resolves to the drive's working directory); UNC shares stay one segment. Duplicate action/sort labels are localised.
• Pure helpers moved to gui/ui/lib.js with Node unit tests (10), run in CI.
• Verified in a browser against a mocked Tauri bridge: scan → history → changes rendering, cancelled path, first-scan hint, rescan, delete → re-render, duplicates toolbar; no console errors.

Docs

README: features, CLI examples (incl. a cron/Task Scheduler one-liner), GUI paragraph, roadmap — drill-up/breadcrumbs ticked (it had shipped without the tick), release binaries described as checksummed + provenance-attested rather than "signed", signed GUI installers listed as the remaining open item.