Add read-only database V2 parser

docs/design/database-v2-parser.md

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+# Database V2 Read-only Parser
+
+## Problem
+
+Crates and files do not explain every orange track in Serato. The legacy
+`database V2` contains Serato's library-level track paths and metadata, so it must
+be inspected independently from crate references.
+
+## Architecture
+
+The parser reads the file as a big-endian tag-length-value stream. Top-level
+`otrk` records contain nested fields including `pfil` (path), `tsng` (title),
+`tart` (artist), `talb` (album), and `tgen` (genre). Text is UTF-16 big-endian.
+
+Analysis reports total database entries, entries whose filenames occur in the
+selected music scan, entries outside that scan, scanned tracks absent from the
+database, and duplicate database paths. “Outside scan” is deliberately not called
+missing because Serato databases can include samples and tracks from other roots.
+
+## Safety
+
+The parser calls only `read_bytes`; it never opens the database for writing. No
+metadata values or personal paths are sent to logs or the dashboard.
+
+## Verification
+
+Synthetic TLV fixtures cover version, paths, metadata, incomplete records, and
+health integration. The sample library includes a generated ten-entry database.