Bootstrap: sync spine footnotes to 4705/4705 (#1492)

[PurHur]

Summary

• Align BootstrapSelfhostBundleTest and README/docs footnotes with canonical bootstrap-spine-count.php (4705/4705).
• Refresh prelinked/bootstrap-gen0/.m3_compiler_lib_sidecar.sha to match current spine entry SHA-1 (stamp-only; gen-0 blob refresh deferred).

Root cause

check-selfhost-spine-count-sync red on master after recent spine/inventory landings: docs still showed 4697/4703 while spine reports 4705/4705.

Verification

php script/bootstrap-spine-count.php
# bootstrap-spine-count: 4705/4705

php script/check-selfhost-spine-count-sync.php
# check-selfhost-spine-count-sync: OK (spine 4705/4705 from compiler_lib_spine_smoke)

php script/check-selfhost-spine-deferred-sync.php
# check-selfhost-spine-deferred-sync: OK (0 deferred: (none); spine 4705/4705)

php script/check-selfhost-spine-sidecar-sync.php
# check-selfhost-spine-sidecar-sync: OK (stamp matches spine entry SHA-1 b09b6cee20738d547f13abb422b5351805e6f534)

./script/docker-exec.sh -- bash -lc 'make bootstrap-selfhost-link'
# bootstrap-selfhost-link: OK /compiler/build/selfhost

Closes #1492 (bootstrap gate drift)

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