storage: add per-page checksums to fact pages and B+tree pages (file format v8) #388

Problem

Only the header has a checksum. src/storage/packed_pages.rs and src/storage/btree_v6.rs store no checksum per page. A torn write, a bad sector, or a bug that writes to the wrong page stays hidden until:

• postcard decoding fails by chance, or
• a query quietly returns wrong data. (bug: after a kill during an active run the EAVT index silently loses most entities' entries while AEVT stays intact — entity-bound lookups return [] and no rebuild path exists #370 was a case of this: rebuilt index entries pointed at the wrong page and slot, and nothing noticed.)

SQLite, LMDB and most embedded stores check each page when they read it, or offer an option to.

Ask

• Add a CRC32 (the crate already uses crc32fast) to every fact page and every B+tree node page. Check it on each read. On a mismatch, return a structured STG-0xx error. Never return data from a page that fails the check.
• This changes the on-disk layout, so it belongs in v8 with the bug: EavtKey and AevtKey missing value bytes causes silent data loss when batching multi-valued facts #287 key change. It should not become a separate v9.
• The v7→v8 migration writes the checksums.
• The verify tool (Add index integrity check (verify) and public rebuild-indexes-from-fact-pages #373) checks every page checksum.

Philosophy check

Aligned: reliability over features, single file. The cost is 4 bytes per 4 KB page and one CRC per page read.