FN-7068: rescue quarantined flaky tests

Rescue quarantined dashboard and engine tests before the deletion deadline.

- Realign the DevServerView mobile CSS test with split mobile rules and balanced at-rule extraction.
- Complete FN-5488 self-healing TaskStore fakes for overlap-scope paths.
- Remove rescued dashboard and engine tests from quarantine configs and the ledger.

Files changed:
 .../__tests__/DevServerView.mobile.test.tsx        | 74 ++++++++++++++++++----
 packages/dashboard/vitest.config.ts                |  8 +--
 ...in-review-merge-stall-deadlock-recovery.test.ts |  7 ++
 ...f-healing-fn-5488-fast-path-regressions.test.ts |  6 ++
 packages/engine/vitest.config.ts                   |  8 +--
 scripts/lib/test-quarantine.json                   | 18 +-----
 6 files changed, 79 insertions(+), 42 deletions(-)

Fusion-Task-Id: FN-7068

Fusion-Task-Lineage: b40a551f-8981-49a1-86a3-660f08cceb94

scripts/lib/test-quarantine.json

@@ -1,20 +1,4 @@
 {
   "$comment": "Flaky-test quarantine ledger (deletion ratchet — see AGENTS.md 'Flaky tests: quarantine on sight' and docs/testing.md 'Quarantine ledger and the deletion ratchet'). A test observed failing without a corresponding real bug is quarantined ON SIGHT: add an entry here AND a matching one-line `exclude` entry in that package's vitest config, in the same commit. Every entry needs a non-empty `reason` (link the failing run) and a `quarantinedAt` date — the entry expires 14 days later, at which point the test file is DELETED unless someone rescues it with evidence it catches real regressions plus a root-cause fix (never appeasement). There is deliberately no loader module and no automation around this file: it is a dated record, the vitest config exclude is the mechanism, and the sweep is policy executed by whoever touches the suite.",
-  "entries": [
-    {
-      "file": "packages/engine/src/__tests__/self-healing-fn-5488-fast-path-regressions.test.ts",
-      "reason": "Failing in CI full-suite shard 1/4: 'expected +0 to be 1' + 'this.store.parseFileScopeFromPrompt is not a function'. Run: https://github.com/Runfusion/Fusion/actions/runs/28206337202",
-      "quarantinedAt": "2026-06-25"
-    },
-    {
-      "file": "packages/engine/src/__tests__/in-review-merge-stall-deadlock-recovery.test.ts",
-      "reason": "Failing in CI full-suite shard 2/4: 'expected FN-5485 to be null'. Run: https://github.com/Runfusion/Fusion/actions/runs/28206337202",
-      "quarantinedAt": "2026-06-25"
-    },
-    {
-      "file": "packages/dashboard/app/components/__tests__/DevServerView.mobile.test.tsx",
-      "reason": "Failing in CI full-suite shard 4/4: 'expected +0 to be 1' (mobile CSS structure assertion). Run: https://github.com/Runfusion/Fusion/actions/runs/28206337202",
-      "quarantinedAt": "2026-06-25"
-    }
-  ]
+  "entries": []
 }