ffi 3.2 B: locked-in golden test for the Obj-C selector mangling table

`examples/ffi-objc-dsl-07-mangling-table.sx` exercises every common
mangling shape in one fixture and pins the resolved selectors via
both `.txt` and `.ir` snapshots:

| sx method                          | derived selector            |
|-----------------------------------|----------------------------|
| `length`                           | `length`                    |
| `addObject(o)`                     | `addObject:`                |
| `combine_and(a, b)`                | `combine:and:`              |
| `insert_after_index(a, b, c)`      | `insert:after:index:`       |
| `add_observer_for_event(a, b, c, d)` | `add:observer:for:event:`   |
| `initWithFrame_options(f, o)`      | `initWithFrame:options:`    |
| `custom_name #selector("actualSelectorName")` | `actualSelectorName` |

The class is synthesized at runtime via `objc_allocateClassPair` +
`class_addMethod` per selector (mirrors the pattern in
`ffi-objc-dsl-{01..05}.sx`), so the test actually dispatches through
the real Obj-C runtime on macOS.

Single commit because the implementation already shipped in 3.0/3.2;
this is a new regression that locks in current behavior, not a
test-then-make-green pair.

The `.ir` snapshot opts in via the existing run_examples.sh mechanism
(presence of a `.ir` file for the same name triggers capture). The
captured `OBJC_METH_VAR_NAME_*` constants surface every selector
string change at a glance.

166/166 tests.

current/CHECKPOINT-FFI.md

@@ -536,9 +536,14 @@ arity-mismatch under the override path downgrades from `.err` to
 `GetMethodID` validates descriptors). Snapshot for
 `ffi-objc-dsl-06-selector-override.sx` flipped to working output.
 
+Phase 3.2 B landed:
+`examples/ffi-objc-dsl-07-mangling-table.sx` exercises 7 mangling
+shapes (niladic, arity 1-4, camelCase across pieces, override) in
+one fixture. Both `.txt` and `.ir` snapshots locked — a change to
+`deriveObjcSelector` produces one diff that surfaces every affected
+case at once via the `OBJC_METH_VAR_NAME_*` constants in the IR.
+
 Open work, in roughly the order they make sense:
-- **Phase 3 step 3.2 — B (golden mangling table)** — locked-in IR
-  fixture for the default mangling rule.
 - **Phase 3 step 3.2 — C1..C5** — uikit.sx migration, one cluster
   per commit, chess regression after each.
   test for the default-mangling table. Escape hatch for selectors