Attribute SQLite database size by object and page usage

[Widthdom]

Summary

A 583 MB DB reports roughly 42 MB free and about 193 MB logical object size, leaving hundreds of MB unattributed without table/index/page/WAL breakdown.

Audit IDs: O-23. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Follow-up adjacent to closed #4338's maintenance diagnostics.

Expected behavior and PR scope

Provide bounded, machine-readable page attribution for tables, indexes, free/reclaimable pages, WAL, and overhead, with a clear unavailable fallback. Keep this diagnostic/read-only.

Implementation guide for Codex 5.6 sol xhigh

• Use SQLite dbstat/page pragmas where supported and define units/rounding explicitly.
• Separate main DB, WAL/SHM, freelist, allocated object pages, internal/overflow, and unexplained residual.
• Apply redaction and byte limits; aggregate/top-N rather than dumping every page.
• Cover empty/small/large DBs, WAL mode, dbstat unavailable, corruption/read-only, and Windows file sizing.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• Reported categories reconcile to file/allocation totals within documented bounds.
• Unavailability is explicit, not zero.
• No diagnostic mutates the database.

[Widthdom]

Work started on branch fix-issue4888.