Self-host: dedupe JitDomSave.php spine require (#1402)

[PurHur]

Summary

• Remove duplicate require_once for ext/dom/JitDomSave.php in compiler_lib_spine_smoke/main.php (introduced when Self-host: sync spine for DOM save/validate AOT helpers #35558/Self-host: add DOM validate/save JIT units to compile spine (#35540) #35559 appended the DOM save helper block).
• Sync spine footnotes and BootstrapSelfhostBundleTest assert to 7959/7959 via spine-sync --footnotes-only.

Fixes check-selfhost-spine-count-sync failure:

compiler_lib_spine_smoke: duplicate require_once for ext/dom/JitDomSave.php — remove the extra line

Closes #1402

Test plan

./script/aot-smoke.sh
# aot-smoke: 8 passed, 0 failed

./script/docker-exec.sh -- bash -lc 'php script/check-selfhost-spine-count-sync.php && php script/check-selfhost-spine-coverage-sync.php'
# check-selfhost-spine-count-sync: OK (spine 7959/7959 from compiler_lib_spine_smoke)
# check-selfhost-spine-coverage-sync: OK (spine covers all 7959 inventory files)

./script/spine-sync.sh --footnotes-only
# step 4: count + coverage + roadmap OK (sidecar stamp step expected red without relink — footnote-only PR)

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