ffi 3.2 A2: implement #selector("explicit:string") override

Make-green half of the cadence step started in A1. Wires the
`#selector` directive end-to-end:

- Lexer token `hash_selector` at src/token.zig + lookup row in
  src/lexer.zig.
- AST field `selector_override: ?[]const u8 = null` on
  `ForeignMethodDecl` (src/ast.zig).
- Parser block in src/parser.zig that mirrors
  `#jni_method_descriptor` — both occupy the same slot after the
  optional `-> ReturnType` and before the body/terminator. Not
  mutually exclusive at parse time.
- LSP semantic-token list (src/lsp/server.zig) updated.
- Lowering: `deriveObjcSelector` returns
  `{ sel, keyword_count, is_override }`. When `is_override` is true,
  the selector string is the user's literal and `keyword_count` is
  the colon count in that literal. Both `lowerObjcMethodCall` and
  `lowerObjcStaticCall` use the result.

Diagnostic policy when override colon-count ≠ call arity:

- Default mangling path: stays an error (`.err`). The user can fix
  the sx-side name to produce the right keyword count.
- Override path: downgrades to a warning (`.warn`). Rationale:
  Obj-C's `objc_msgSend` doesn't validate colon-vs-arg the way JNI's
  `GetMethodID` validates the descriptor — the runtime dispatches
  regardless and the wrong-arity case becomes silent calling-
  convention corruption. The compiler is the last line of defense
  for this typo class, but the warning preserves the override's
  escape-hatch character (deliberate mismatches still proceed).

Snapshot for `examples/ffi-objc-dsl-06-selector-override.sx` flips
from the pre-3.2 parser-error to working output:

  static override non-null: true

The mismatch diagnostic text in
`examples/ffi-objc-dsl-04-mismatch.sx`'s snapshot is updated to
drop the "once that lands (3.2)" phrasing now that 3.2 is here.

165/165 example tests.

src/ast.zig

@@ -554,6 +554,7 @@ pub const ForeignMethodDecl = struct {
     return_type: ?*Node, // null = void
     is_static: bool = false, // true for `static name :: ...`
     jni_descriptor_override: ?[]const u8 = null, // `#jni_method_descriptor("(Sig)Ret")` — JNI runtime only
+    selector_override: ?[]const u8 = null, // `#selector("explicit:string")` — Obj-C runtime only (Phase 3.2)
     body: ?*Node = null, // sx-side implementation (defined-class only). null = `;`-terminated decl referencing inherited / external method.
 };