Make the source pass the CI lint

The central pipeline lints where the old inline one did not, and it
aborts on any warning. Nothing here was actually wrong: 166 of the 243
warnings were the TIC-80 API and the game's own globals being undeclared,
which is what .luacheckrc is for — the same shape impostor already uses.

The rest was trailing whitespace on 14 lines, now stripped. INPUT_KEY_X
is unused but kept: it belongs to the LEFT/RIGHT/A/B/X/Y set and removing
it would leave a gap in the mapping, so luacheck is told to allow the
family.

Verified locally by merging the sources the way the pipeline does and
running luacheck over the result: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.luacheckrc

@@ -0,0 +1,30 @@
+-- .luacheckrc
+-- Configuration for luacheck
+--
+-- The CI lint step aborts on any warning, so both the project's own globals
+-- and the TIC-80 API functions this game calls have to be declared here.
+
+globals = {
+  -- the game's own globals
+  "Context",
+  "init_game",
+
+  -- TIC-80 callbacks and API
+  "TIC",
+  "btn",
+  "btnp",
+  "cls",
+  "exit",
+  "keyp",
+  "mget",
+  "mset",
+  "rect",
+  "rectb",
+  "spr",
+}
+
+-- The INPUT_KEY_* constants are a coherent set (LEFT/RIGHT/A/B/X/Y); some are
+-- not bound yet, and dropping them would leave gaps in the mapping.
+ignore = { "211/INPUT_KEY_.*" }
+
+std = "lua51" -- TIC-80 runs Lua 5.1