[gspark-etri]

Background

In the #41 cluster check, a prefill pod did not take over the idle decode artifact. That was expected, but the log could not show why (#49).

Changes

• PullStats.BaseBest: the bytes the best offered base keeps at the same offsets, whether it was taken or not. chooseBase now returns its best candidate together with the decision.
• The materializer logs when nothing is taken over:
  • no artifact to take over (k in use, j used within <rebase-idle>);
  • took over nothing: of n idle artifacts the best keeps x of y B at the same offsets (p %, 50 % needed).
• busy is split out of idle.
• deploy/csi/README.md: the Delta apply: turn an idle artifact on the node into the next variant, writing only what changed #41 cluster results:
  • a decode B8 pod was Running 12 s after creation, taking over the idle B2 artifact (15.0 GB left in place, 28.3 MB read, 7.5 s pull, byte-identical, read-only), against about a minute before;
  • an artifact in use was not touched;
  • a prefill pod took over nothing.

Verification

• Store tests: BaseBest >= Reused when a base is taken, and below half when none qualifies.
• CSI tests check both log lines: the recently used case, and two idle artifacts that share nothing with a third.
• The full suite, gofmt and vet are clean on the node.
• Cluster check of Delta apply: turn an idle artifact on the node into the next variant, writing only what changed #41 at 5e69bdd (by the CSI test run):

step	pod Running after	pull	check
decode B2, cold	160 s	10.20 GB from the store	identical
decode B8, B2 idle for 1 min	12 s	took over B2: 15.0 GB in place, 28.3 MB read, 7.5 s	sha256 identical; read-only
decode B2 while the B8 pod runs	69 s	no takeover (B8 in use); 10.20 GB from the node cache	identical
prefill C128, B8 idle	82 s	no takeover; 1.9 GB store + 7.5 GB cache	identical

Cleanup was verified: no mounts, leases or files left, and the CSIDriver is deleted.

Compatibility and limits

• A new stats field and new log lines only.

Related Issue

Closes #49

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