Bootstrap: sync spine count footnotes to 4691/4691 (#1492)

[PurHur]

Summary

• Align BootstrapSelfhostBundleTest spine assertSame with canonical bootstrap-spine-count.php SSOT (4691/4691).
• Refresh README + docs footnotes from stale 4691/4692 and 4689 pairs after recent spine landings (Bootstrap: sync spine for ReflectionFunction::getNumberOfRequiredParameters (#18325) #18360, Bootstrap: sync spine for BuiltinInternalDefaultValues (#18356) #18384, Bootstrap: refresh gen-0 sidecar stamp after spine drift (#1492) #18397).

Root cause

check-selfhost-spine-count-sync red on master: test hardcoded 4689 while spine reports 4691; docs still showed 4691/4692 inventory pair after inventory recount settled at 4691/4691.

Verification

php script/bootstrap-spine-count.php
# bootstrap-spine-count: 4691/4691

php script/check-selfhost-spine-count-sync.php
# check-selfhost-spine-count-sync: OK (spine 4691/4691 from compiler_lib_spine_smoke)

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

php script/bootstrap-inventory.php --check
# OK 4691/4691

make north-star5-verify-fast
# north-star5-verify: OK (fast)

make bootstrap-selfhost-link
# bootstrap-selfhost-link: OK

Refs #1492

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