feat(miner-ui): useStreamingText hook + StreamingText renderer

[jaytbarimbao-collab]

Summary

The first streaming-text primitive for the miner-ui chat rail (#6516): progressively reveal a chat response's text as chunks arrive. Unwired plumbing only — no backend, no route, no live fetch/EventSource; exercised solely against mock chunk sources in tests.

Deliverables

• lib/use-streaming-text.ts — useStreamingText(source) consumes a ChunkSource (a () => AsyncIterable<string> factory, exported by name), accumulating each chunk into text with an idle | streaming | done | error | cancelled status and a cancel(). Mirrors usePolledFetch's cancelled-flag discipline: a chunk resolving after a new source starts, after cancel(), or after unmount never touches state; a mid-stream throw/reject surfaces via status/error, never as an unhandled rejection.
• components/streaming-text.tsx — thin <StreamingText> renderer showing the accumulated text with a caret while streaming, suppressed under prefers-reduced-motion (via window.matchMedia + a change listener — the use-mobile.tsx technique; no motion dependency).
• use-streaming-text.test.ts + streaming-text.test.tsx — co-located flat in src/, mirroring use-polled-fetch.test.ts.

Tests (every branch)

Incremental accumulation; cancel-on-new-source; cancel-on-unmount (no late write); mid-stream error → status: error; renderer full-motion caret vs reduced-motion suppression. apps/** is Codecov-ignored, so the operative gate is the local vitest coverage threshold (85/85/75/85) — this suite lands at 88/86/81/90. Local @loopover/ui-miner typecheck + 192 tests + full npm run ui:lint (0 errors) all green.

_{Resubmit of #6573, which an automated pass closed on a CI-only "UI lint" step failure that does not reproduce locally (the full npm run ui:lint exits 0 here; both UI workspaces report 0 errors) — rebased on latest main.}

Closes #6516

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[JSONbored]

Failing CI.