Add canonical UID-scoped storage primitives

[gabeosx]

Stack position

Depends on the upstream protocol prerequisite: rustmailer#337

This draft exists in the fork so GitHub shows only this layer's diff. It should be retargeted/reopened upstream after the protocol prerequisite merges.

Scope

• exact raw-message blob batches with post-write readback verification
• canonical raw lookup that remains compatible with legacy blobs
• source-scoped UID identity and deduplication
• bounded projection of envelopes and attachments
• deletion cascade updates for the additional exact-raw key

This layer does not connect to UIDONLY servers, select accounts, alter ordinary IMAP routing, or expose a setting.

Verification

• cargo check -p bichon-core
• all 61 blob unit, durability, fuzz, and integration tests
• storage/projection/dedup coverage included in the final 38-test UIDONLY suite