[2/2] feat: let a host annotate the generate action

[dazzatronus]

Stacked PR [2/2]. The mechanical extraction is already split out into #170 (refactor: lift generation type and prompt helpers into shared utils), which is this PR's base branch. This PR contains no refactoring: git diff <base>...HEAD touches nine files, src/core/shared/ai-asset-utils.ts is not among them, and the refactor commit is not in this PR's commit range. Merge #170 first, then retarget this to main.

Studio renders a clip's model and options but the host has no way to say anything about them. This adds one event out and one method in.

Edit emits generation:configChanged when the selected prompt-bearing clip's model, options, length or prompt changes, carrying { clipId, type, model?, options, length, prompt }. The host replies with edit.setGenerationStatus(clipId, { text, tone }), rendered beside Generate.

Non-obvious:

• length is a required key typed number | undefined. It is undefined only for auto, where the duration comes from media that does not exist yet; the resolver's placeholder would otherwise be reported as fact.
• Only tone: "error" disables Generate. A stale host value should not block work the backend would accept.
• A status for a clip that is not selected is stored but not shown, so a late reply cannot paint the wrong clip.
• The note slot already carried "Generates on render"; with no generator registered that copy still wins over any host status.
• TimelineUpdated is a trigger so a live merge-field edit re-announces the resolved prompt.

Verify: npm run verify:ci

Risk: 393 lines, roughly half tests. Inert until a host opts in — no behaviour changes without a registered handler and a status call.