mz334: eliminate and squash fully cached stages

[mzihlmann]

Fixes #334

With the cache lookahead and inferred cross-stage keys in place, a multistage build still built every stage even when all its consumers were served from cache. This PR adds the payoff: after the precompute pass the stage graph optimization from MakeKanikoStages runs again, this time over the precompute stage builders. A COPY --from that resolved via its inferred key is a CachingCopyCommand and no longer counts as a reference, so stages that only fed cached copies get eliminated and newly linear FROM chains get squashed. In the fully warm case a build that forked through COPY --from collapses into a single stage where every layer comes from cache and nothing is unpacked.

Two design points make this safe. First, the composite cache is now recursive: every key is folded into a fixed-size state and the state is the canonical cache key, like OCI chainIDs or git commits. A stage boundary is a resumable digest, so squashed and unsquashed chains produce identical keys by construction and the cache-state-dependent squash cannot change any key. This also fixes the latent separator collision of the joined-text scheme and shrinks the redirect pointer payload to a digest. It busts every existing cache once, which needs a loud release-notes entry. Second, one memoized layer cache is shared between the precompute and the build pass, so every pointer resolution and layer retrieval the precompute made is answered from memory during the build. Nothing is retrieved twice and the build cannot fail on a pointer or layer that vanished in between, which matters because an eliminated stage cannot be rebuilt. The key-match assert stays for resolutions the build pass makes fresh, those copies kept their source stage alive so the files are there to verify against.

The plan renders after the pass, so it shows what will actually be built. dockerfile.go is deliberately untouched apart from exporting Squash, the pass mirrors its loops inline so the two can be merged in a follow-up. This lands as one PR for verification and will be split up for review.

[codecov]

Codecov Report

❌ Patch coverage is 81.34328% with 25 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/executor/build.go	81.06%	18 Missing and 7 partials ⚠️

📢 Thoughts on this report? Let us know!