Implement insert_asset_clip, the one timeline op that could not run

[dallasbpeters]

Every other timeline operation rearranges what is already there: trim narrows a
clip, duplicate_clip copies one, move_clip reorders them, drop_range cuts a
span out. None of them can introduce footage. So an agent asked to put a sting on
the front or cut to the B-roll had no operation that could do it — and the one
that was supposed to, insert_asset_clip, existed only as a line in the schema.
It could be described, parsed, and then silently not performed.

Worth being precise, because I had this wrong myself at first: the editor's
drag-and-drop insert has always worked. insertClipAt is implemented and wired to
the timeline's drop handler. It was the agent path that had nothing behind it.

Where it lives, and why

The recipe already existed twice over. useTimeline's drop handler inlined it —
splice, resequence, rederive — while the comment on its own moveClip explains
why that belongs in document/timeline.ts: two façades over one recipe is how the
two drift into disagreeing about clip widths and anchored pills. So the pure
insertAssetClip goes there, the op dispatcher calls it, and the drop handler now
delegates rather than keeping a copy.

Details worth stating

• Neighbours, not an index. The schema names beforeClipId/afterClipId,
  which is the right contract for a model: "after the intro" survives the list
  changing under it and a number does not.
• It returns the new clip's id beside the document, unlike its neighbours here
  which return a bare one, because the caller needs it — the drop handler selects
  what it inserted and corrects its length when the duration probe lands. Stapling
  the id onto the document would put a field on it the schema does not have, and
  that document gets parsed, and saved.
• An unprobed asset takes the placeholder duration, not zero:
  resequenceClips floors a clip at 0.001s, which is a clip you cannot see,
  select or drag.
• The clip is marked agent. The timeline shows the origin, and "user" would
  make a model's proposal look like a cut somebody made on purpose.
• It throws rather than doing nothing quietly — unknown asset, unknown
  neighbour, empty source range. Doing nothing quietly is what this op did for its
  entire existence.

Verification

305 tests pass across 18 files, six of them new. Checked against the real
schema: a sting inserted at the front pushes the interview to 4s; a trimmed
B-roll insert (source 40–50s of a 90s file) lands at the right place; the document
stays documentSchema-valid throughout; all three refusals fire.