M1-4: Length axis (TDD)

- Add axis.js: length axis rectangle state, 28 tests
- 71/71 unit tests green

WORKER-CONTEXT.md

@@ -0,0 +1,36 @@
+# SlabUploader M1 — shared build context (for :202 worker tasks)
+
+REPO: /home/ty/SlabUploader  — work ONLY inside /home/ty/SlabUploader/frontend.
+Branch: M1-dev-rev4 (already checked out; DO NOT switch branches, DO NOT push).
+This is Milestone 1 (Capture) of a slab-measuring web app. Browser-only, NO backend,
+NO inference, NO OpenCV/ONNX. Pure JS (not TS). Svelte 5 + SvelteKit + adapter-node.
+
+TEST COMMANDS (run these, show real output):
+  unit:  cd /home/ty/SlabUploader/frontend && npx vitest run
+  e2e:   npx playwright test --list   (collection only; full e2e needs the M1-6 container)
+
+EXISTING (already committed, green, DO NOT break):
+  src/lib/core/mask.js   -> keySheet(data,{sheet,sensitivity}), maskStats(mask)
+  src/lib/core/math.js   -> scale, sqft, bdft, roundHalfUp, sampledWidths,
+                            sampledWidthsFrom, snapToEighth
+  tests/unit/mask.test.js, tests/unit/math.test.js  (24 tests, all pass)
+  src/routes/+page.svelte (placeholder home — M1-3 replaces it)
+  src/routes/+layout.svelte, src/routes/api/health/+server.js
+
+TDD IS MANDATORY (test-driven-development skill):
+  1. Write the failing test(s) FIRST.
+  2. Run them and WATCH them fail for the right reason (feature missing, not a typo).
+  3. Write the MINIMAL code to pass.
+  4. Run them and confirm they pass.
+  5. Run the FULL unit suite (npx vitest run) and confirm no regressions + pristine output.
+  6. Only then are you done. Commit is done by the coordinator — you do NOT commit.
+
+CONVENTIONS:
+  - Tab indentation (the repo uses tabs).
+  - Pure logic lives in src/lib/core/*.js (DOM-free, unit-testable under node).
+  - Svelte components live under src/lib/ (e.g. src/lib/components/).
+  - Every new exported function needs a unit test that you SAW fail first.
+  - No console.log left in production code.
+  - Do not add dependencies.
+  - Report at the end: files created/changed, the exact test command output, and
+    any place you had to make a judgment call.