RedDB bodies via BlobStore pointer, drop inline truncation (ADR-0006)

[filipeforattini]

What to build

The RedDB storage backend stops inline-truncating bodies: it writes bytes to the BlobStore and stores only a cas pointer plus metadata, bytes-first then pointer-second, removing the ~1 MiB coverage ceiling. See ADR-0006.

Acceptance criteria

• The RedDB backend writes bodies to the BlobStore and stores a cas pointer, not inline bytes.
• Bodies above the former inline cap are stored in full (no truncation flag).
• Write order is bytes-first, pointer-second (a crash leaves an orphan blob, never a dangling pointer).
• Tests cover large-body round-trip and pointer integrity.

Blocked by

• BlobStore trait + content-addressed fs backend (ADR-0006) #63
• Content identity + non-destructive content dedup (ADR-0006) #64
• Concrete StateStorage + host-intel read-back (ADR-0008) #68