Cart grammar: tolerate leading blank lines before the magic header

`extras: $ => []` in the cart grammar made the parser fail at byte 0
on any whitespace-only or empty line before `pico-8 cartridge //...`.
Real PICO-8 carts always start with the header at byte 0 so this
rarely surfaced in production, but it ( a ) broke the `tree-sitter
test` corpus harness, which prepends a newline to each fixture, and
( b ) would mis-flag a hand-edited cart that gained an accidental
blank line up top.

Fix: prefix the `cartridge` rule with `repeat($._blank_line)` and add
a hidden `_blank_line` token matching `[ \t]*\n`. Junk content before
the header ( a non-blank, non-magic line ) still produces an ERROR.

Restores the test corpus that was dropped in v0.1 ( previously failing
on this same edge case ) and adds a fixture for the unknown_section
fallback while the corpus is being rebuilt.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

grammars/p8-cart/grammar.js

@@ -18,16 +18,31 @@
 module.exports = grammar({
   name: 'p8_cart',
 
-  // Whitespace is significant inside hex sections, so we don't skip it.
+  // Whitespace is significant inside hex sections, so we don't skip it
+  // globally. Tolerance for stray leading blanks before the magic header
+  // is added explicitly via the `repeat($._blank_line)` at the top of
+  // `cartridge` ( see below ).
   extras: $ => [],
 
   rules: {
     cartridge: $ => seq(
+      // Tolerate stray whitespace / blank lines before the magic header.
+      // Real PICO-8 carts begin with the header on byte 0, but allowing
+      // a leading run of blanks ( a ) lets the `tree-sitter test` corpus
+      // framework, which prepends a newline to each fixture, run cleanly
+      // and ( b ) keeps the parser robust against a hand-edited cart that
+      // gained an accidental blank line up top.
+      repeat($._blank_line),
       optional($.header),
       optional($.version),
       repeat($.section),
     ),
 
+    // A line that has no content other than horizontal whitespace and a
+    // newline. Hidden ( underscore prefix ) so it does not appear in the
+    // syntax tree.
+    _blank_line: $ => token(/[ \t]*\n/),
+
     header: $ => /pico-8 cartridge \/\/[^\n]*\n/,
     version: $ => /version[ \t]+\d+\n/,