fix(input): Key.Up/Down/Left/Right are the held set's 128-131

The arrow Key constants folded to the codes of w/s/a/d, which is what the
per-frame key reports for an arrow, but cocoa.ll has always stored an arrow in
the held-key set under 128-131. So Input.key_down(Key.Up) read the W bit and
Input.move_i's arrow half never moved anything. Input.key keeps its WASD alias,
so games comparing it with 'w' (snake, chronorift) still take the arrows.

New example input_arrows.ludic, checked by ludic-dev test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/library/input_arrows.ludic

@@ -0,0 +1,30 @@
+# input_arrows.ludic — the arrow keys in the held-key set. Key.Up/Down/Left/Right are
+# 128-131, the codes the platform stores an arrow under; they used to fold to w/s/a/d,
+# so Input.key_down(Key.Up) read the W bit and Input.move_i ignored the arrows. Driven
+# headless by injection, the same way the #50 device tests are.
+#
+# Deterministic; a full run prints:  128 129 130 131 1 0 -1 1 0
+program InputArrows {
+  function bi(b: bool) -> int { if b { return 1 };  return 0 }
+
+  entry {
+    print(Key.Up)
+    print(Key.Down)
+    print(Key.Left)
+    print(Key.Right)
+
+    Input.press(Key.Up)                  # hold the up arrow
+    Input.poll()
+    print(bi(Input.key_down(Key.Up)))    # 1 — the arrow is held
+    print(bi(Input.key_down('w')))       # 0 — and it is not W
+    let m = Input.move_i()
+    print(m.y)                           # -1 — move_i takes the arrows as well as WASD
+
+    Input.release(Key.Up)
+    Input.press(Key.Right)
+    Input.poll()
+    let r = Input.move_i()
+    print(r.x)                           # 1
+    print(bi(Input.key_down(Key.Up)))    # 0 — released
+  }
+}