docs: run only the test legs a change can break

A new rule in CLAUDE.md: weigh which legs the change touches, run each once
when its input is final, no up-front baseline round. Reclaimed the four lines
it costs by tightening the commit-message and documenting-a-change rules.

Claude-Session: https://claude.ai/code/session_01Wzs2qG6Y5qm7mKmJHfu44G

CLAUDE.md

@@ -27,12 +27,9 @@ node scripts/showcase/run.mjs     # rebuild docs/showcase/ (sample .odt/.docx +
 ```
 
 Tests live in `tests/` (outside `src/`, so `svelte-check` ignores them), jsdom via Vitest.
-`roundtrip.test.ts` covers the ODF export↔import round trip + a foreign-doc/style-resolver leg;
+`roundtrip.test.ts` covers the ODF export↔import round trip + a foreign-doc/style-resolver leg; `lo-roundtrip.test.ts` re-saves through LibreOffice and `lo-fuzz.test.ts` reads the fuzz seeds back through it (`LO_SEEDS`, `LO_DUMP=<file>` for triage) — both **self-skip** without `soffice`, so `npm test`/CI stay green;
-`lo-roundtrip.test.ts` re-saves through LibreOffice and `lo-fuzz.test.ts` reads the fuzz seeds back through it (`LO_SEEDS`, `LO_DUMP=<file>` for triage) — both **self-skip** without `soffice`, so `npm test`/CI stay green;
+`corpus.test.ts` round-trips the committed `tests/corpus/` documents (by `make-fixtures.mjs` + Word re-saves in `corpus/word/`, never our exporter); `fuzz-roundtrip.test.ts` round-trips seeded random documents under random export options (`fuzzDoc.ts`, `fuzzOptions.ts`) through both formats and validates every export against the schemas (`schemaValidate.ts`; `FUZZ_SEEDS=500` widens it);
-`corpus.test.ts` round-trips the committed `tests/corpus/` documents (by `make-fixtures.mjs` + Word re-saves in `corpus/word/`, never our exporter);
+`schema-validation.test.ts` validates the `kitchenSink.ts` exports against the vendored schemas in `tests/schemas/` (self-skips without `xmllint`); `package-lint.test.ts` checks the semantic invariants schemas can't express (unique ids, dangling references); `tests/unit/` holds fast helper tests; the five browser runs (`tests/smoke/`, `tests/dom/`, `tests/layout/`, `tests/monkey/`, `tests/tabs/`) share `tests/browser.mjs`. All test tooling stays a `devDependency`. No linter/formatter. CI (`.github/workflows/ci.yml`) runs `check` + `test`.
-`fuzz-roundtrip.test.ts` round-trips seeded random documents under random export options (`fuzzDoc.ts`, `fuzzOptions.ts`) through both formats and validates every export against the schemas (`schemaValidate.ts`; `FUZZ_SEEDS=500` widens it);
-`schema-validation.test.ts` validates the `kitchenSink.ts` exports against the vendored schemas in `tests/schemas/` (self-skips without `xmllint`); `package-lint.test.ts` checks the semantic invariants schemas can't express (unique ids, dangling references); `tests/unit/` holds fast helper tests;
-the five browser runs (`tests/smoke/`, `tests/dom/`, `tests/layout/`, `tests/monkey/`, `tests/tabs/`) share `tests/browser.mjs`. All test tooling stays a `devDependency`. No linter/formatter. CI (`.github/workflows/ci.yml`) runs `check` + `test`.
 
 ## Rules
 
@@ -42,13 +39,12 @@ the five browser runs (`tests/smoke/`, `tests/dom/`, `tests/layout/`, `tests/mon
 - **Never describe how the current code differs from an older version** (no "previously…", "this used to…", "changed from…"). Comment only what the current code does and why — git history covers the rest.
 - **Don't use Word as a placeholder for "a word processor".** Where LibreOffice does the same thing, describe the behaviour itself ("the caret moves", "the zone auto-grows") instead of "Word-style" / "like Word" / "as in Word". Name a product only where the statement really is about that product: its file format (`w:tblLook`, DOCX), or a quirk only it has — then name both if both apply.
 
-**Commit messages** — a clear description, never an essay: a subject line plus **at most ~8
+**Commit messages** — a subject line plus **at most ~8 lines**, however large the change. Probed
-lines**, however large the change. Probed behaviour, measurements and rationale lists belong
+behaviour, measurements and rationale belong in `docs/architecture/` or the nearest `CLAUDE.md`.
-in `docs/architecture/` or the nearest `CLAUDE.md`, not repeated in the history.
 
-**IMPORTANT: no real-world document's name anywhere in the repo, and a commit message
+**IMPORTANT: no real-world document's name anywhere in the repo, and a commit message names no
-names no document at all** — not even "the thesis": describe the fix and its measurement,
+document at all** — not even "the thesis": describe the fix and its measurement, never the file it
-never the file it helped. Detail in `tests/render-parity/README.md`.
+helped. Detail in `tests/render-parity/README.md`.
 
 **IMPORTANT: never introduce a default only this editor has.** Both importers suppress values equal
 to the defaults, so an editor-only default is indistinguishable from a failed style resolution and
@@ -68,15 +64,19 @@ silently lands in every imported document as direct formatting. The defaults fol
 fetches an x86-64 Chrome that cannot run on arm64). Recipe and the three-engine CI matrix in
 `docs/headless-testing.md`; driving the live app is the only way to verify rendering or NodeViews.
 
+**Running tests** — weigh which legs the change can actually break and run only those: `npm test`
+(or the one file) for logic, the LO legs for export/import, the browser runs for rendering,
+`test:parity` for layout. Each leg once, at the point its input is final — no up-front baseline
+round, no re-run of a leg that is still green.
+
 **Naming** — components `PascalCase.svelte`, every `.ts` module `camelCase`; extension files are
 named by feature (`image.ts`, `indent.ts`), not `XyzExtension.ts`.
 
 **Documenting a change** — a feature that changes how this codebase behaves gets documented where
 it loads on demand, not here. **This file** only grows for a new command, a new hard rule, or a new
 top-level directory; everything else is one line in the nearest directory `CLAUDE.md`, or a section
-in its `docs/architecture/` file. Write only what the code can't say — probed Word/LibreOffice
+in its `docs/architecture/` file. Write only what the code can't say — probed behaviour, sentinel
-behaviour, sentinel order, constants that must agree across files. Keep this file under 120 lines:
+order, cross-file constants. Keep this file under 120 lines: a section past ~5 lines moves out.
-a section growing past ~5 lines belongs somewhere else.
 
 ## Source layout