Adds core/archipelago/src/blobs.rs: a SHA-256 content-addressed store
that writes bytes to ${data_dir}/blobs/<cid> with a sibling <cid>.meta
JSON file (mime, filename, size, created_at, optional tiny thumbnail).
BlobStore::put is idempotent, max 64 MiB per blob, and issues HMAC-SHA256
capability tokens scoped to (cid, peer_pubkey_hex, expiry_epoch). Tokens
are verified in constant time and rejected on expiry. This is the
foundation piece for the mesh ContentRef typed envelope — the /blob/<cid>
HTTP route and ContentRef variant will land in a follow-up increment
once the HMAC key is plumbed from node identity.
No consumer yet, so the module compiles with dead_code warnings; these
will clear when the HTTP handler and ApiHandler state wiring land next.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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