docs: add TASK-140 for parallel test runner

Full test suite takes several minutes sequentially.
Task specifies options for parallel execution:
- GNU parallel
- xargs -P
- Makefile with dependencies

Expected speedup: 3+ minutes -> ~25-50 seconds

Author: subtleGradient

.tasks/backlog/TASK-140-parallel-test-runner.md

@@ -0,0 +1,100 @@
+# TASK-140: Add parallel test runner for faster CI
+
+## Priority: P2 (SECONDARY)
+
+## Summary
+
+The full test suite in `zig/harness/` takes a long time to run sequentially. 
+Implement a parallel test runner to speed up CI and local development feedback loops.
+
+## Current State
+
+- ~40 test scripts in `zig/harness/test-*.sh`
+- Each test is independent (uses temp DBs, no shared state)
+- Sequential run takes several minutes
+- Each individual test takes 2-10 seconds
+
+## Files to Modify
+
+- `zig/harness/run-all-tests.sh` (new file)
+- `zig/Makefile` (add `test-parallel` target)
+
+## Acceptance Criteria
+
+1. [ ] Create `run-all-tests.sh` that runs tests in parallel
+2. [ ] Use GNU parallel or xargs -P for parallelism
+3. [ ] Collect and summarize results at end
+4. [ ] Exit non-zero if any test fails
+5. [ ] Support `--jobs N` or `PARALLEL_JOBS` env var
+6. [ ] Default to `nproc` or 4 concurrent jobs
+7. [ ] Show progress (e.g., "12/40 tests completed")
+8. [ ] Preserve individual test output for debugging failures
+
+## Implementation Options
+
+### Option A: GNU Parallel
+
+```bash
+#!/usr/bin/env bash
+find . -name 'test-*.sh' -type f | \
+  parallel --jobs ${PARALLEL_JOBS:-$(nproc)} \
+           --bar \
+           --results .tmp/test-results/{/} \
+           bash {}
+```
+
+### Option B: xargs
+
+```bash
+#!/usr/bin/env bash
+find . -name 'test-*.sh' -type f | \
+  xargs -P ${PARALLEL_JOBS:-4} -I{} bash -c '
+    echo "Running {}"
+    bash {} > .tmp/test-output-$(basename {}).log 2>&1
+    echo "Exit code: $?"
+  '
+```
+
+### Option C: Makefile with dependencies
+
+```makefile
+TESTS := $(wildcard test-*.sh)
+test-parallel: $(TESTS:.sh=.result)
+%.result: %.sh
+	@bash $< > .tmp/$@.log 2>&1 && touch $@ || (cat .tmp/$@.log; exit 1)
+```
+
+## Considerations
+
+1. **Build contention**: Multiple tests may try to rebuild Zig extension concurrently
+   - Solution: Add `zig build` as prerequisite before parallel tests
+   
+2. **Temp file collisions**: Tests create temp DBs in `.tmp/`
+   - Current: Uses `mktemp` so no collision
+   - Verify each test uses unique temp files
+
+3. **Output interleaving**: Parallel output can be confusing
+   - Solution: Capture per-test output, show only on failure
+
+4. **Resource limits**: Too many concurrent SQLite processes
+   - Solution: Default to 4, allow override
+
+## Expected Speedup
+
+With 40 tests at ~5 seconds each:
+- Sequential: ~200 seconds (3+ minutes)
+- Parallel (4 jobs): ~50 seconds
+- Parallel (8 jobs): ~25 seconds
+
+## Parent Docs / Cross-links
+
+- Gap Analysis: `research/zig-cr/97-test-gap-analysis.md`
+- Current runner: `zig/harness/test-parity.sh` (sequential)
+
+## Progress Log
+
+- 2024-12-20: Created task card
+
+## Completion Notes
+
+(To be filled upon completion)