Surface collapsed child thread errors

[brsbl]

What was wrong

Collapsed hierarchy rows derive their status from getCollapsedChildActivity, but parented threads are deliberately excluded from the ordinary unread-completion model. That policy correctly kept successful child completions quiet, but it also discarded a terminal child failure, leaving the parent unmarked while the errored child row was hidden.

What changed

• Carry a parented thread's terminal error status into the collapsed hierarchy's existing unreadError signal and destructive CircleX treatment.
• Preserve the existing policy for successful parented children, so healthy collapsed hierarchies remain unmarked.
• Add core status-model and rendered ProjectRow regressions for collapsed errored and healthy direct children.
• No wire, daemon protocol, CLI, SDK, or persistence contracts changed.

How you verified

• Confirmed the new core regression failed before the production change (unreadError was false) and passed afterward.
• pnpm exec turbo run test --filter=@bb/client-core --force -- test/thread-activity.test.ts — 39 passed.
• pnpm exec turbo run test --filter=@bb/app --force -- src/components/sidebar/ProjectRow.interactions.test.tsx — 11 passed.
• pnpm exec turbo run typecheck --filter=@bb/client-core --filter=@bb/app --force — passed.
• Drove the exact branch web app at PR head b6b43dcc4357e2d34c62bab9a652a27af0bc33a5 in Chrome for Testing 151 across 390×844, 768×900, 1440×900, and 3440×1440. Each viewport passed five expand/collapse cycles, responsive transition widths, client navigation, and hard reload with zero page errors.
• Repeated the primary flow in Safari 26.5.2: collapsed errored and healthy states, expansion, five cycles, client navigation, and hard reload all passed with zero page errors.

Before — merge base 42d2c1bec

The errored direct child is hidden, but its collapsed parent has no failure indicator.

After — PR head b6b43dcc4

The same fixture, route, interaction state, and viewport now show the existing failure indicator on the collapsed parent.

Fixes # — no linked issue.

BB-Thread-ID: thr_764udexes3

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