Archive the 2026-10-19 execution handoff

[CompleteDotTech]

Closes #299

Preserves the supplied 2026-10-19 execution handoff under orchestration/handoffs/2026-10-19/, including its ledgers, proposed updates, historical checks and supporting evidence. The overview labels the snapshot as historical and keeps the proposed actions distinct from completed work.

All 31 source-manifest entries were verified before import. The original manifest and ZIP digest are retained. An import record documents 141 explicit high-entropy omissions across nine source files and one trailing-blank-line normalization, with original/imported digests and source offsets; the imported snapshot has its own valid manifest. The supplied ZIP is unchanged.

Validation completed locally:

• Reconstructed every imported file from the original ZIP and the recorded transformations; verified all 35 staged blobs and the exact scope.
• Ruff 0.17.0 lint and format checks.
• Hygiene: 604 tracked files scanned.
• Generated catalog check.
• Fixtures: 51 folders, zero failures.
• Signed commit verified locally.

The required GitHub workflows exercise both Python test versions and notebook execution. Independent Opus review is required before merge. This PR changes only the handoff directory and does not apply the archived GitHub proposals.