feat(authoring): add closed public journey intake contract

[777genius]

Adds the closed public journey input contract and a separate prepared authenticated bridge/runner/checker path, preserving existing private and offline fixture contracts. C3a intentionally cannot accept a live journey or completed E. Python authenticated entrypoints also reject before effects until C3b supplies an independently provisioned controller; a receipt-selected interpreter cannot authenticate itself.

Stacked on #216 after #230. Refs #204, #208. Exact head 210c77b; eight owned paths, 999 changed lines (996 additions, 3 deletions). Prepared command, custody and terminal logic is retained for complete C3b execution, installer/observer validation, workflow and promotion admission.

Independent hosted Astra low review found one P1 in interpreter authority. The 70-line follow-up correction received independent ACCEPT at this exact head. The original review ran 33 passing tests; the follow-up independently ran all 20 Python tests, including the new four-case regression, plus a bounded diagnostic confirming zero subprocess calls. There were no test skips. Sealed artifacts and source preservation were verified; 3377 paths outside the fix remain unchanged. All ten native, packed, source-acquisition and polyglot CI checks passed on this exact head, including both Windows architectures.

Positive interface tests remain explicitly synthetic. No genuine public/native E2E, authenticated release custody, installer/scanner assessment or release qualification is claimed. Complete C3b, N2 acceptance and all remaining plan/release gates remain mandatory. Useful YAML functionality, dependencies and assets are preserved. No main activation or public release is authorized by this checkpoint.

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