ffi 3.2 A1 (xfail): add #selector("...") override regression test

Phase 3.2 xfail half. `#selector("explicit:string")` is the escape
hatch for cases where the sx-side method name doesn't conveniently
produce the target selector under the default mangling rule
(Phase 3.0 — split on `_`, each piece becomes a keyword with a
trailing `:`).

Surface form mirrors `#jni_method_descriptor("(Sig)Ret")` — sits
after the optional `-> ReturnType` and before the method body /
terminator.

Test fixture covers both lowering paths:

- Static method override: `NSObject.gimme()` with override
  "description" — exercises lowerObjcStaticCall (Phase 3.1).
- Instance method override: `NSDictionary.lookup(self, key)` with
  override "objectForKey:" — declared (parse + AST + lowering
  wiring) but not invoked at runtime (no real NSDictionary in
  scope). The declaration alone locks in the multi-arg-override path.

Pre-3.2: parser doesn't know `#selector`; snapshot captures
"expected ';'" at the override site, exit=1. Next commit (A2) wires
the lexer token, AST field, parser block, and lowering integration;
snapshot flips to working output.

165/165 example tests. Plan at
`~/.claude/plans/lets-see-options-for-merry-dijkstra.md`.

tests/expected/ffi-objc-dsl-06-selector-override.exit

@@ -0,0 +1 @@
+1

tests/expected/ffi-objc-dsl-06-selector-override.txt

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+/Users/agra/projects/sx/examples/ffi-objc-dsl-06-selector-override.sx:20:26: error: expected ';'