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/src/grammar.json

@@ -5,6 +5,13 @@
     "cartridge": {
       "type": "SEQ",
       "members": [
+        {
+          "type": "REPEAT",
+          "content": {
+            "type": "SYMBOL",
+            "name": "_blank_line"
+          }
+        },
         {
           "type": "CHOICE",
           "members": [
@@ -38,6 +45,13 @@
         }
       ]
     },
+    "_blank_line": {
+      "type": "TOKEN",
+      "content": {
+        "type": "PATTERN",
+        "value": "[ \\t]*\\n"
+      }
+    },
     "header": {
       "type": "PATTERN",
       "value": "pico-8 cartridge \\/\\/[^\\n]*\\n"