perf: retire duplicate nested docs HTML objects

[hannesrudolph]

Retire the redundant nested HTML objects from the docs R2 artifact while retaining both public URL forms. The compatible Worker from #260 serves /route/index.html from /route, with legacy fallback.

The preparer omits a nested HTML entry only when exactly one canonical alias refers to the same hashed source and every manifest field other than the storage key matches, including all HTTP/custom metadata. Missing, ambiguous or mismatched aliases retain both objects. Physical index files remain for previews. Full publication uses the existing manifest cleanup; partial uploads keep legacy keys.

Compatible reader verified: Worker 1e1d3eb1-e61e-4d97-9caf-c0e57de329c9 was deployed from d493cce3b3c4531aa7234303b1c7ffd4c2f923cc by Pages run 37099918404, successfully completed at 2026-10-03 05:29:01 UTC. The served 39-request matrix passed canonical/explicit GET/HEAD, locale, dotted/encoded, root/404, asset and Markdown checks. Proof SHA256: 1e766554296b2ebdc724f5ab2d94dc6190e9a747c7701312b88fc1672d89e769. Keep this PR draft until its required CI is green and fresh head/base guards pass. No manual bucket deletion or overlapping publication. The hosting runbook documents rollback: restore dual-key artifacts on current main before an older Worker, using the dual-key generator baseline 60a3699e2d39047044bd11d99f64b758efde8417 while preserving current docs.

Validation:

• Full Node suite: 294 pass, 1 optional SDK skip; structured code review clean.
• Full local build/smoke: 15,459 pages, 62,665 objects (external source-code index excluded).
• Frozen corpus: 91,197 -> 62,665 objects; exactly 28,532 redundant nested keys omitted, zero retained object changes. Every omitted key has identical retained metadata/bytes, and every physical file was rehashed and preserved.
• Real builder/Worker tests retain canonical and explicit HTML URLs, GET/HEAD, dotted/encoded/localized routes, Markdown redirects and asset contracts.
• Dry-run full upload deletes only the expected duplicate key; partial upload retains it.

The local 28,532 count is a frozen-corpus measurement. The production receipt will report the actual count from its current docs snapshot.