Make watch mode rerun affected tests

docs/watch-mode.mdadded+45-0

Changes to docs/watch-mode.md

		@@ -0,0 +1,45 @@
	1	+# Watch mode
	2	+
	3	+Watch mode runs the full discovered suite once, then polls Zig source and test
	4	+files after the configured debounce interval. Start it with a test directory:
	5	+
	6	+```bash
	7	+zig-test --test-dir tests --watch
	8	+```
	9	+
	10	+Type `r` and press Enter at any time to request an explicit full-suite rerun.
	11	+Ctrl-C stops the watcher and any UI server cleanly.
	12	+
	13	+## Affected-test selection
	14	+
	15	+Each rerun prints both the selected test and the reason it was selected:
	16	+
	17	+```text
	18	+Watch selection:
	19	+ - math.test.zig (imports changed source: src/math.zig)
	20	+```
	21	+
	22	+The selection rules are conservative:
	23	+
	24	+- A modified or newly added test file runs by itself. Multiple changed tests
	25	+ are batched by the debounce interval and run together.
	26	+- For a source change, the watcher follows relative `.zig` imports from every
	27	+ discovered test, including transitive imports, and runs tests whose import
	28	+ graph contains the changed file.
	29	+- Named module imports such as `` cannot be resolved from
	30	+ source text alone. If any test has unresolved imports, or a changed source
	31	+ file maps to no test, the watcher safely runs the full suite.
	32	+- Deleting a test, or renaming one (observed as a delete plus an add), runs the
	33	+ full suite so reporters and the live UI receive the new test topology.
	34	+- Deleting or renaming an imported source file also falls back to the full
	35	+ suite when its dependency graph can no longer be resolved.
	36	+
	37	+The watcher scans the project root for `.zig` changes while using `--test-dir`,
	38	+`--pattern`, and recursive-discovery settings to decide which files are tests.
	39	+It excludes Git metadata, Zig caches/output, `node_modules`, and `.codex`.
	40	+
	41	+## Debouncing
	42	+
	43	+`--watch-debounce <ms>` controls the polling and batching interval. The default
	44	+is 300 milliseconds. A test run is never interrupted: edits made while tests
	45	+are running are detected on the next poll and cause another affected run.