* fix(studio): show the current artifact's source after switching artifacts
The canvas source view feeds one Streamdown a fence built from the selected
artifact's code, but never keys it. Streamdown does not revise a block it has
already committed, so the panel keeps rendering the previous artifact's source.
Key the source view on the artifact ID plus a hash of its code: tool artifact
IDs are derived from the tool call, not the code, so the ID alone does not
change when a tool artifact is updated in place.
* Name the real root cause and make the source-key test load-bearing
The remount is needed because Streamdown memoizes a fenced code block on its
hast node's line/column span, which ignores the text inside the fence, so two
canvases of equal line count compare equal and the old source stays on screen.
Verified in Chromium against streamdown 2.5.0: unkeyed, 70 lines -> 70 lines
renders the previous artifact, 70 -> 71 and 70 -> 90 render correctly.
Move the key expression into the source branch so it costs nothing while the
artifact is streaming and the view is unmounted, and export the helper from
types.ts so the test exercises the shipped code instead of a local copy of the
formula (it passed before even with the key removed from the component).
* Assert the source view's Streamdown key wiring, not just the helper
The suite exercised buildArtifactSourceKey but never the component, so deleting
key={buildArtifactSourceKey(artifact)} from the Streamdown left every test
green. There is no DOM renderer available to these tests, so parse
artifact-surface.tsx with the TypeScript compiler API (already a devDependency)
and assert the source view's Streamdown carries that key.
Mutation-checked: removing the key fails 1 test, swapping it for artifact.id
fails 1, and making the helper ignore code fails 2.
* Tighten the comments added by this PR
a0a3a7b24a