Require structured download completion evidence

[fly1d]

Summary

• add a required, language-neutral completion_requirements.download field to both full and compact planner schemas
• carry download requirements through plan review, fast paths, execution guards, and trusted continuations without granting tools or bypassing download permission
• accept success only after browser-reported completed download evidence; correlate timed-out downloads with later list_downloads verification by download ID
• emit a content-free trace note when download intent corrects an inconsistent requires_state_change: false planner response
• keep Chrome and Firefox behavior mirrored and wait for completion in download_resource_from_page

Behavior

Real save/download requests now require a completed file before done can report success. Requests that only find a URL, link, button, instructions, or explanation remain read-only. Edited reviewed plans clear stale download requirements when their steps change.

This metadata only tightens completion evidence. It does not authorize download tools, switch modes, or bypass the existing capability/host permission gate.

Tests

• node test/run.js (1649 passed)
• npm run test:security (60/60)
• npm run test:toolbar-guard (33 passed)

Closes #2752

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