fix(host): harden migrated host boundaries

[wolfiesch]

What this changes

• Bind artifact digest verification and returned chunks to the same opened file identity, with bounded reuse of verified identities.
• Make image-upload spool setup recover from interrupted initialization, and make cleanup fail closed unless marker, root identity, and file identity prove ownership.
• Exclude workspace archive from opening or live external runtimes, seal quiescent managed workspaces before deletion, and release terminal ownership if buffered output cannot be published.
• Gate legacy transcript images and generic artifacts by their own host features, keep unavailable media snapshots referentially stable, preserve complete artifact metadata in exports, and make inline/attachment presentation and verification labels match the descriptor.
• Rename the compatibility proof to legacy-bridge-continuity, run it as a required CI leg for the exact T4 head, validate both the provenance repository and SHA before checkout, and retain machine-readable evidence. The gate verifies the exercised T4 client continuity flow against the pinned legacy OMP appserver; it does not test deployment of @t4-code/host-service.

This is the post-merge hardening follow-up to #90. PR #90 merged while these fixes were being prepared.

How you verified it

• pnpm check
• pnpm test
• pnpm --filter @t4-code/host-service test
• pnpm test:legacy-bridge-continuity with the pinned Lycaon OMP source
• node --test scripts/check-release-consistency.test.mjs

Local verification used both Node 25.2.1 and Bun 1.3.14; the pnpm test run includes Bun-backed package tests. Node 25.2.1 emitted the expected engine warning; CI is pinned to the required Node 24.13.1 runtime.

Checklist

• pnpm check passes (release contract, provenance, lint, typecheck)
• pnpm test passes; new behavior has a test that fails without this change
• Any pasted logs or screenshots are redacted: no secrets, tokens, pairing codes, or private paths
• Ported code (T3 Code / OMP) follows the provenance rules in THIRD_PARTY_NOTICES.md

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, add credits to your account and enable them for code reviews in your settings.