New `src/ir/jni_descriptor.zig`:
- `writeType(allocator, buf, ctx, type_node)` appends one JNI
descriptor for an sx type AST node.
- `deriveMethod(allocator, ctx, method)` returns the full
`(args)ret` descriptor for a `ForeignMethodDecl`, skipping the
implicit `self` for instance methods.
- `Context.enclosing_path` resolves `*Self` to its `L<path>;` form.
Primitive table: void→V, bool→Z, s8/u8→B, s16→S, u16→C, s32→I,
s64→J, f32→F, f64→D. Arrays: `[]T` / `[*]T` / `[N]T` → `[<elem>`.
`*Self` → `L<enclosing>;`. Cross-class `*Foo` → explicit
`CrossClassRefNotYetSupported` error (lands in step 2.9 with the
ForeignClassDecl registry lookup).
Tests in `src/ir/jni_descriptor.test.zig`: primitive table coverage,
void-on-null, *Self, slice, cross-class-fail-fast, plus three
deriveMethod scenarios (instance, static, multi-param, slice param).
Step 2.8 is internal compiler work — derivation isn't observable at
the sx surface until call-site lowering at step 2.11. The cadence
rule's xfail-then-green pattern presupposes a snapshot harness that
doesn't apply to internal-only functions; the rule re-applies at
2.11 where end-to-end observation returns.
zig build test passes; 126/126 examples still green.
