fix(reads): keep heavy worker failures off the gateway thread

[BYK]

Closes #1736. Part of #1708.

What changed

• Make heavy read worker failures explicit: optional reads degrade while required knowledge and distillation reads return a retryable preparation error.
• Prevent unavailable, timed-out, and broken worker paths from rerunning heavy scans synchronously on the gateway thread.
• Preserve frozen knowledge and durable prompt deltas when context-source FTS fails, and make direct compaction retryable when required reads are unavailable.
• Add regression coverage for worker failures, distillation prewarm, context selection, vector reads, and gateway/compaction behavior.

Validation

• Focused read-offload, LTM context, pipeline streaming, and compaction tests: 219 passed.
• Core/gateway/OpenCode/Pi/UI TypeScript checks, lint and formatting: passed.
• Full Vitest run: 11,279 passed, 233 skipped, 3 failed. The failing tests involve this environment's pnpm wrapper trying an unattended dependency reinstall (fixture and test-shard subprocesses) and absent optional ONNX GPU assets; bundle-exports could not build for the same pnpm reason. No changed-path tests failed.
• Adversarial correctness review found and fixed a failed-FTS/pinned-context gap, with regression coverage.
• Devin's Layer 4 refresh findings: tests prove the worker failure returns 503 and an unsent first-injection delta is reselected on retry. Both regressions failed before their fixes; 182 gateway tests, gateway typecheck, lint, and formatting passed.

The current Git tree hash matches the validated local tree (3c192911372716c1e8a0afe05bdb9833657512ae).

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[github-actions]

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[github-actions]

Coverage Results 📊

✅ Patch coverage is 84.78% (78 of 92 changed executable lines covered; target 80%).
Project statement coverage is 84.49% (up 0.01 percentage points from base (e6ec679) to head (a1e1b60)).

Changed files with executable lines (11)

File	Patch coverage	Changed executable lines
packages/gateway/src/pipeline.ts	60.71%	17/28 covered; missed: 3914, 3930, 19823, 19824, 19825, 19826, 19827, 19831, 19834, 19835, 19838; partial branches: 19299, 19665, 19669
packages/core/src/ltm.ts	100.00%	13/13 covered; partial branches: 2474, 3324, 3430, 3669, 3732
packages/core/src/embedding/search.ts	70.00%	7/10 covered; missed: 39, 61, 62; partial branches: 38
packages/core/src/lat-reader.ts	100.00%	3/3 covered; partial branches: 386
packages/core/src/recall.ts	100.00%	1/1 covered; partial branches: 323
packages/core/src/db.ts	100.00%	4/4 covered
packages/core/src/entities.ts	100.00%	1/1 covered
packages/core/src/gradient.ts	100.00%	1/1 covered
packages/core/src/read-offload.ts	100.00%	12/12 covered
packages/core/src/temporal.ts	100.00%	1/1 covered
packages/core/src/vector-pool.ts	100.00%	18/18 covered

Coverage diff

@@            Coverage Diff             @@
##          main     #1890       +/-##
==========================================
+ Coverage    84.48%    84.49%    +0.01%
==========================================
  Files          304       304         —
  Tracked lines     47640     47700       +60
  Branches     38639     38697       +58
==========================================
+ Hits         40247     40302       +55
- Misses        7393      7398        +5
- Partials      4154      4160        +6

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