Stop tracking workspace/; bundled-demos/ is the canonical demo source

`workspace/` is runtime state (per-user folders, no-auth dev's `code/`)
and shouldn't be in git. The same files were previously committed under
both `workspace/code/` and `src/static/bundled-demos/`, which forced a
Docker `diff -q` sync check and leaked user-scoped paths into version
control.

- /workspace/ added to .gitignore; all previously tracked files removed
  via `git rm --cached`.
- src/static/bundled-demos/ becomes the single source of truth: panel16
  demos, led_tutorial, led_patterns, neopixel demos, and main.py move
  here alongside the existing canonical demos.
- New BUNDLED_DEMOS_DIR config; user_workspace seeders read from it.
- main.py lifespan seeds WORKSPACE_ROOT/code/ on startup so a fresh
  clone running `pipenv run dev` still gets the full sample set
  (existing files never overwritten — user edits survive restarts).
- Dockerfile drops `COPY workspace` and the diff sanity check.
- README/LED_TUTORIAL repointed at the new canonical paths.
- test_led_patterns loads led_patterns.py from bundled-demos.
- test_api uses mkdir(exist_ok=True) for `code/` (startup pre-creates).

Co-authored-by: Cursor <cursoragent@cursor.com>

src/static/bundled-demos/panel16_matrix_rain.py

@@ -0,0 +1,37 @@
+"""16x16 matrix-style rain animation."""
+
+from machine import Pin
+import neopixel
+import random
+import time
+
+from panel16_utils import PANEL_H, PANEL_W, clamp8, xy_to_index
+
+
+np = neopixel.NeoPixel(Pin(4), PANEL_W * PANEL_H)
+rng = random.Random(42)
+
+heads = [rng.randrange(-PANEL_H, 0) for _ in range(PANEL_W)]
+
+for _frame in range(320):
+    for y in range(PANEL_H):
+        for x in range(PANEL_W):
+            np[xy_to_index(x, y)] = (0, 0, 0)
+
+    for x in range(PANEL_W):
+        heads[x] += 1
+        if heads[x] > PANEL_H + 6:
+            heads[x] = rng.randrange(-PANEL_H, 0)
+
+        head_y = heads[x]
+        for tail in range(8):
+            y = head_y - tail
+            if 0 <= y < PANEL_H:
+                brightness = clamp8(255 - tail * 36)
+                np[xy_to_index(x, y)] = (0, brightness, 0)
+
+    np.write()
+    time.sleep(0.045)
+
+np.fill((0, 0, 0))
+np.write()