CacheBackend trait with tiered local + remote and read/write modes

[calvin-archastro]

Epic: Aster restores real build artifacts from a shared cache, including the GitHub Actions cache
Task: CacheBackend trait with tiered local + remote and read/write modes

Local git hooks skipped: pre-push on the submission push and the publish push. CI is the gate for what those hooks check.

Problem and author intent

ArchDev could not generate review prose. This ready-for-review PR preserves the verified branch so the exact diff can be reviewed.

What changed

Changed paths include src/cache/artifact_cache.rs, src/cache/backend.rs, src/cache/mod.rs. Review the commits and files directly.

Scope

Determine backend, frontend, or full-stack scope from the diff.

Risk assessment

Risk is unclassified because generated review metadata was unavailable.

User impact

Determine user impact from the diff.

Testing

Review the daemon job output for the checks executed against this exact head.

Follow-ups and known issues

Semantic hunk metadata is absent; reviewers must inspect the complete diff.

[calvin-archastro]

ArchDev is watching this pull request. State: watching (cycle 5 of 5 for head 7f5fdb4).

when	trigger	decision	result
2026-10-18T12:19:03.944Z	review feedback	enqueue job_1b1f8765403670a1531bfae3	published c4dc300
2026-10-18T12:33:50.342Z	verify head	enqueue job_876853522618eb101f8a1f7b	no change
2026-10-18T23:41:23.149Z	merge conflict	enqueue job_b70d42cc91005c5a1e5f8a0c	published 606dcc5
2026-10-18T23:58:13.002Z	review feedback	enqueue job_2f87e0860b9879c4b634296e	published 7f5fdb4
2026-10-19T00:16:35.280Z	verify head	enqueue job_91d2d6381ca2179830b078b7	no change

Paused by the no-autofix label or a draft; never resolves reviewer threads. Jobs: job_1b1f8765403670a1531bfae3, job_876853522618eb101f8a1f7b, job_b70d42cc91005c5a1e5f8a0c, job_2f87e0860b9879c4b634296e, job_91d2d6381ca2179830b078b7.