fix(chat): keep thinking <details> open while streaming

ChatView keyed the transcript each-block by object identity, but core/chunks
returns new RenderedChunk objects per delta, so Svelte recreated each
<article>/<details> every frame — an opened Thinking element snapped shut on
the next token. Key by stable identity instead (c${seq} for committed, p${i}
for append-only provisional) so streaming reuses the DOM. Adds a regression
test that the <details> stays open across a streaming update.

Verified: svelte-check 0/0, vitest 222, biome clean, build ok.

0 files changed, 0 insertions, 0 deletions