feat: add ban-words check for underscore-prefixed Zig struct fields

This adds a new validation to ban-words.test.ts that detects Zig struct
fields starting with underscore and recommends using # instead for private
fields. The Zig compiler has been extended to support this # prefix syntax!

- Detects patterns like `_foo: u32,` in struct definitions
- Shows friendly message recommending `#foo` instead of `_foo`
- Allows existing 148 instances to maintain backwards compatibility
- New violations will fail the test with helpful guidance

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test/internal/ban-limits.json

@@ -17,6 +17,7 @@
   "EXCEPTION_ASSERT(!scope.exception())": 0,
   "JSValue.false": 0,
   "JSValue.true": 0,
+  "^\\s*_[a-zA-Z_][^:]*:.*[,=]": 148,
   "alloc.ptr !=": 0,
   "alloc.ptr ==": 0,
   "allocator.ptr !=": 1,

test/internal/ban-words.test.ts

@@ -38,6 +38,7 @@ const words: Record<string, { reason: string; regex?: boolean }> = {
   "!= alloc.ptr": { reason: "The std.mem.Allocator context pointer can be undefined, which makes this comparison undefined behavior" },
 
   ": [^=]+= undefined,$": { reason: "Do not default a struct field to undefined", regex: true },
+  "^\\s*_[a-zA-Z_][^:]*:.*[,=]": { reason: "Use `#` instead of `_` to denote private struct fields in Zig (e.g., `#foo` instead of `_foo`). The Zig compiler has been extended to support this feature!", regex: true },
   "usingnamespace": { reason: "Zig 0.15 will remove `usingnamespace`" },
 
   "std.fs.Dir": { reason: "Prefer bun.sys + bun.FD instead of std.fs" },