Adopt pinned public core snapshot and prepare Sized 2.0.0

docs/UPGRADING-2.md

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+# Moving from Sized 1.0 to 2.0
+
+The existing flags and table/CSV/NDJSON workflows remain. Update automation to
+handle incomplete scans: exit 0 means a complete result; exit 1 means a missing
+root, an incomplete scan or another operation failure. CLI argument errors use
+Clap's exit 2. An incomplete scan may still produce useful output. Diagnostics
+go to stderr, and each JSON report includes `complete`.
+
+Hard-linked files now contribute allocated blocks once, owned by the first path
+in deterministic traversal order. Apparent sizes remain per pathname. Totals
+may be lower than 1.0 for this reason. Ignore rules inherit through directories;
+`--one-file-system` explicitly skips other mounted filesystems. Allocation is
+filesystem-reported blocks, not guaranteed recoverable space.
+
+## Rust consumers
+
+The CLI package is version 2.0.0. Its scanner is the exact `sized-core` revision
+recorded in `CORE-SNAPSHOT.json`, also used by SizeQueen. The core snapshot's
+internal package version remains 0.0.0; the full Git revision and file hashes
+identify it. It is an included path dependency, so no private forge access or
+separate core installation is required.
+
+Public scanner types and `sized::scan` are re-exported from that dependency.
+`EntryType` adds `Special` and `Unknown`; update exhaustive matches. `Node` has
+additional accounting/status fields and internal state; obtain nodes through
+`scan_tree` instead of struct literals. `Args` adds `one_file_system`, and
+`run` returns scan completeness as a boolean. `build_tree` remains a compatibility
+helper, but use `scan_tree` to retain structured errors and diagnostics.
+
+Use a fresh `ScanControl` per scan. Cancellation is cooperative between
+filesystem calls. SizeQueen does not launch the CLI; both use the core in process.