sync: plain mirrors earn durability evidence; unsatisfiable offload_requires is a config error

[mbertschler]

Problem (friction log F21, S1 — confirmed live)

The mirror sync path contains no vector-advance call at all, so offload_requires naming a mirror destination waits forever. In the walk, the reference config's laptop gated on the crypt-mirror cloudbox — not refused, not warned, just a gate that can never open, discovered only when offload was finally attempted. (#121 later added an offload-time pre-check that catches locally configured crypt mirrors; the walk hit the peer-relayed blind spot tracked by #145.)

Decision (Martin, 2026-07-24 — option 3a)

• Plain (non-crypt) mirrors earn evidence: a mirror sync that ran the full BLAKE3 --checksum comparison is content-verified end-to-end and advances the vector with method rclone-blake3. (Shallow runs and crypt mirrors advance nothing, as today.)
• Unsatisfiable requirements are config errors: offload_requires naming a locally-configured destination that can structurally never gate (crypt mirror) is rejected at config load — fail-early instead of wait-forever or offload-time-only. The error points at CA/packed layouts (and plain-mirror-with-verification) as alternatives.

Plan

• Advance the vector in the mirror success path when (and only when) the run's comparison was full BLAKE3 (rclone-blake3 method; the gate already accepts content-verified methods).
• Move/duplicate the CanEverGateOffload check from the offload pre-check (offload: fail fast when a required target can never satisfy the gate #121) to config resolution for locally-known names; unknown (peer-relayed) names stay permitted and remain offload fail-fast: catch peer-relayed required targets that can never gate #145's territory.
• Tests: mirror-verified advance; shallow/crypt advance nothing; config load rejects crypt-mirror requires with the documented message.

Reference-setup amendment for this already landed in design/reference-setup.md ("Offload gates deliberately name only targets that produce durability evidence"); update it again to re-admit plain mirrors once this ships.

See design/friction-log.md F21.