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/test/corpus/cart.txt

@@ -0,0 +1,109 @@
+==================
+empty cart skeleton
+==================
+
+pico-8 cartridge // http://www.pico-8.com
+version 42
+__lua__
+__gfx__
+__map__
+__sfx__
+__music__
+
+---
+
+(cartridge
+  (header)
+  (version)
+  (section (lua_section (lua_marker)))
+  (section (gfx_section (gfx_marker)))
+  (section (map_section (map_marker)))
+  (section (sfx_section (sfx_marker)))
+  (section (music_section (music_marker))))
+
+==================
+cart with lua content
+==================
+
+pico-8 cartridge // http://www.pico-8.com
+version 42
+__lua__
+function _draw()
+ cls()
+end
+__gfx__
+00000000
+
+---
+
+(cartridge
+  (header)
+  (version)
+  (section
+    (lua_section
+      (lua_marker)
+      (lua_content
+        (line)
+        (line)
+        (line))))
+  (section
+    (gfx_section
+      (gfx_marker)
+      (body
+        (line)))))
+
+==================
+lua identifier resembling section marker
+==================
+
+pico-8 cartridge // http://www.pico-8.com
+version 42
+__lua__
+local __foo__ = 1
+local s = "__lua__"
+__gfx__
+00
+
+---
+
+(cartridge
+  (header)
+  (version)
+  (section
+    (lua_section
+      (lua_marker)
+      (lua_content
+        (line)
+        (line))))
+  (section
+    (gfx_section
+      (gfx_marker)
+      (body
+        (line)))))
+
+==================
+unknown section name
+==================
+
+pico-8 cartridge // http://www.pico-8.com
+version 42
+__lua__
+__future_section__
+opaque body
+__gfx__
+00
+
+---
+
+(cartridge
+  (header)
+  (version)
+  (section (lua_section (lua_marker)))
+  (section
+    (unknown_section
+      (section_marker)
+      (body (line))))
+  (section
+    (gfx_section
+      (gfx_marker)
+      (body (line)))))