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>

changes/arrow-keys.md

@@ -0,0 +1,9 @@
+bump: patch
+type: fix
+**Held arrow keys register** — `Key.Up`, `Key.Down`, `Key.Left` and `Key.Right` are 128-131.
+
+They folded to the codes of w, s, a and d, which is what the single per-frame key
+(`Input.key`) reports for an arrow. The held-key set has always stored an arrow under
+128-131, so `Input.key_down(Key.Up)` tested the W bit and `Input.move_i`'s arrow half never
+moved anything. `Input.key` keeps its WASD alias: a game comparing it with `'w'` still
+takes the arrows.

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
+  }
+}

selfhost/backend/stdlib/emit_key.ludic

@@ -1,7 +1,12 @@
 # emit_key.ludic — `Key.Name` -> the key code the input layer delivers, folded at
 # compile time (like Color.*). Letters are the lowercase character (what the held-key
-# set stores), digits their ASCII, plus the specials. Arrow keys alias WASD because the
+# set stores), digits their ASCII, plus the specials; Escape is 27 (0.3, #88).
-# platform layer maps them that way; Escape is 27 (0.3, #88).
+#
+# The arrows are 128-131, the codes the platform stores them under in the held-key set
+# (cocoa.ll's ev_keyval). They used to fold to w/s/a/d, which is what the single
+# per-frame key (Input.key) reports for an arrow - but the held set never had those bits
+# for an arrow, so Input.key_down(Key.Up) and Input.move_i's arrow half never fired.
+# Input.key keeps its WASD alias, so a game comparing it with 'w' still takes the arrows.
 function key_lookup(name: pointer) -> int {
   if (name == "Space")     { return 32 }
   if (name == "Enter")     { return 10 }
@@ -11,10 +16,10 @@ function key_lookup(name: pointer) -> int {
   if (name == "Shift")     { return 16 }
   if (name == "Tab")       { return 9 }
   if (name == "Backspace") { return 8 }
-  if (name == "Up")        { return 119 }
+  if (name == "Up")        { return 128 }
-  if (name == "Down")      { return 115 }
+  if (name == "Down")      { return 129 }
-  if (name == "Left")      { return 97 }
+  if (name == "Left")      { return 130 }
-  if (name == "Right")     { return 100 }
+  if (name == "Right")     { return 131 }
   if (name == "Digit0") { return 48 }
   if (name == "Digit1") { return 49 }
   if (name == "Digit2") { return 50 }

selfhost/ludicc.seed.ll

@@ -61197,7 +61197,7 @@ ifend23:
   %t27 = icmp ne i32 %t26, 0
   br i1 %t27, label %then24, label %ifend26
 then24:
-  store i32 119, ptr %retval
+  store i32 128, ptr %retval
   br label %ret
 ifend26:
   %t28 = load ptr, ptr %t0
@@ -61205,7 +61205,7 @@ ifend26:
   %t30 = icmp ne i32 %t29, 0
   br i1 %t30, label %then27, label %ifend29
 then27:
-  store i32 115, ptr %retval
+  store i32 129, ptr %retval
   br label %ret
 ifend29:
   %t31 = load ptr, ptr %t0
@@ -61213,7 +61213,7 @@ ifend29:
   %t33 = icmp ne i32 %t32, 0
   br i1 %t33, label %then30, label %ifend32
 then30:
-  store i32 97, ptr %retval
+  store i32 130, ptr %retval
   br label %ret
 ifend32:
   %t34 = load ptr, ptr %t0
@@ -61221,7 +61221,7 @@ ifend32:
   %t36 = icmp ne i32 %t35, 0
   br i1 %t36, label %then33, label %ifend35
 then33:
-  store i32 100, ptr %retval
+  store i32 131, ptr %retval
   br label %ret
 ifend35:
   %t37 = load ptr, ptr %t0