fix(workspace): preserve active conversation indicators

[roxi3906]

Problem

Two workspace indicators can become inconsistent with the underlying runtime state:

• Answering an ask-user prompt can clear the thinking/tool indicator when a provider stop and the continuation tool arrive together, or when a delayed command response races a newer terminal snapshot.
• The scroll-to-bottom button can remain visible at the native scroll limit because Chromium reports fractional layout geometry beyond integer scroll metrics.

Change

• Keep Main-owned prompt authority projected after incremental event processing without restarting the first-output waiting edge.
• Guard post-drain restoration with an ownership generation and reject stale snapshot authority while still delivering its unseen events.
• Route elicitation snapshot synchronization through the public runtime facade and apply the shared revision gate to ordinary and revised responses.
• Use a 0.5px scroll-edge tolerance, small enough to preserve deliberate scrollbar release while ignoring subpixel residue.
• Add regression coverage for same-batch stop/continuation events, stale live and elicitation snapshots, asynchronous ownership release, architecture boundaries, and fractional bottom geometry.

Testing

• Rebased onto upstream/main at 8a2f032a5; git range-diff confirms the feature patch is unchanged.
• 10 focused Vitest files: 516 passed.
• npm run typecheck:web: passed.
• Touched-file ESLint: passed with zero errors (one file is ignored by the repository configuration); git diff --check: passed.
• npm run build:e2e: passed.
• Scroll Electron E2E before the rebase: 4 passed; 2 existing anchor assertions failed with the same values reproduced on an untouched main baseline (266.6875 and 36). No related scroll implementation or spec changed between that baseline and the current upstream base.
• Full Vitest suite was not run locally: the affected-test planner expands to full because message-scroller.test.tsx has no module owner; local validation remains scoped to the requested indirect related set.
• Post-rebase upstream CI is running for 0e7175201. The upstream fix for the previously observed Notebook admission-test timeout is included in the new base.