ffi 3.0: inst.method(args) DSL dispatch on #objc_class receivers

Implementation half of the cadence step started in the previous commit.
`lowerForeignMethodCall` for `#objc_class` / `#objc_protocol` runtimes
no longer bails; it routes through a new `lowerObjcMethodCall` helper
that derives the Obj-C selector from the sx method name and lowers to
`objc_msg_send` against the cached SEL slot (same intern path as
explicit `#objc_call`).

Default selector mangling (matches clang's keyword-method convention):
- Niladic (arity 0 excluding self): name verbatim. `length()` → "length".
- Arity ≥ 1: split the sx method name on `_`; each piece becomes a
  keyword with a trailing `:`. `addObject(o)` → "addObject:";
  `combine_and(a, b)` → "combine:and:";
  `initWithFrame_options(f, o)` → "initWithFrame:options:".

Arity validation: keyword count (pieces from the `_`-split) must equal
call-site arity excluding self. Mismatch diagnoses at the call site
with a hint pointing at the forthcoming `#selector("...")` override
(Phase 3.2) for selectors that don't fit the underscore-split rule.

Mangling helper `deriveObjcSelector` and dispatch helper
`lowerObjcMethodCall` sit alongside `lowerForeignMethodCall`. The
existing fall-through diagnostic for non-JNI/non-Obj-C runtimes
remains for Swift (Phase 4 territory).

Tests `examples/ffi-objc-dsl-{01-niladic,02-one-arg,03-multi-keyword,
04-mismatch}.sx` snapshots flip from the pre-3.0 bail diagnostic
(exit=1) to working output (exit=0 for cases 01-03) and the specific
keyword-count mismatch diagnostic for case 04. Each test follows the
established pattern from `ffi-objc-call-08-multi-keyword.sx`:
synthesize a class at runtime via `objc_allocateClassPair` /
`class_addMethod`, declare a matching `#objc_class`, invoke the DSL
form. 163/163 tests; chess unaffected (JNI dispatch path untouched).

tests/expected/ffi-objc-dsl-01-niladic.exit

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

tests/expected/ffi-objc-dsl-01-niladic.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-objc-dsl-01-niladic.sx:36:14: error: method calls on 'objc_class' runtime not yet supported (Phase 3/4)
+length = 42

tests/expected/ffi-objc-dsl-02-one-arg.exit

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

tests/expected/ffi-objc-dsl-02-one-arg.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-objc-dsl-02-one-arg.sx:28:14: error: method calls on 'objc_class' runtime not yet supported (Phase 3/4)
+addObject(21) = 42

tests/expected/ffi-objc-dsl-03-multi-keyword.exit

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

tests/expected/ffi-objc-dsl-03-multi-keyword.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-objc-dsl-03-multi-keyword.sx:27:14: error: method calls on 'objc_class' runtime not yet supported (Phase 3/4)
+combine_and(7, 42) = 742

tests/expected/ffi-objc-dsl-04-mismatch.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/ffi-objc-dsl-04-mismatch.sx:18:14: error: method calls on 'objc_class' runtime not yet supported (Phase 3/4)
+/Users/agra/projects/sx/examples/ffi-objc-dsl-04-mismatch.sx:18:14: error: Obj-C selector for 'SxProbeMismatch.something_extra' has 2 keyword(s) but the call passes 1 argument(s); split the sx method name on '_' so it produces exactly 1 keyword(s), or override with `#selector("...")` once that lands (3.2)