Bootstrap: refresh gen-0 compiler_lib sidecar stamp after spine drift (#1492)

[PurHur]

Summary

• Updates prelinked/bootstrap-gen0/.m3_compiler_lib_sidecar.sha to match current compiler_lib_spine_smoke/main.php SHA (00962fc5…, was stale ae5d54d6…).
• Refreshes manifest.json generated_at via bootstrap-gen0-refresh-sidecar.
• Fixes script/docker-exec.sh tar-fallback sync-back: merge prelinked/bootstrap-gen0/ with docker cp …/. instead of nesting bootstrap-gen0/bootstrap-gen0/ (exited containers cannot docker exec for path classification).

Unblocks M4 bootstrap-loop-gen2-recompile-spine / make bootstrap-loop-probe when native parseAndCompile returns null but gen-0 sidecar fallback is valid.

Test plan

php script/check-selfhost-spine-sidecar-sync.php
# check-selfhost-spine-sidecar-sync: OK (stamp matches spine entry SHA-1 00962fc5…)

./script/docker-exec.sh -- bash -lc './script/bootstrap-loop-gen2-recompile-spine.sh'
# bootstrap-loop-gen2-recompile-spine: OK gen-2=… gen-3=… (3760/3760 spine, argv -o)

./script/docker-exec.sh -- bash -lc 'make bootstrap-loop-probe'
# bootstrap-loop-probe: M4 full ladder OK — gen-1→gen-2 native + gen-2→gen-3 spine + full-revision (exit 0)

Relates to #1492, #8703, #8704.

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