ffi 2.8: JNI descriptor derivation + table-driven Zig unit tests

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.

src/ir/ir.zig

@@ -39,6 +39,8 @@ pub const type_bridge = @import("type_bridge.zig");
 pub const resolveAstType = type_bridge.resolveAstType;
 pub const bridgeType = type_bridge.bridgeType;
 
+pub const jni_descriptor = @import("jni_descriptor.zig");
+
 pub const types_tests = @import("types.test.zig");
 pub const inst_tests = @import("inst.test.zig");
 pub const module_tests = @import("module.test.zig");
@@ -47,6 +49,7 @@ pub const interp_tests = @import("interp.test.zig");
 pub const lower_tests = @import("lower.test.zig");
 pub const type_bridge_tests = @import("type_bridge.test.zig");
 pub const emit_llvm_tests = @import("emit_llvm.test.zig");
+pub const jni_descriptor_tests = @import("jni_descriptor.test.zig");
 
 test {
     @import("std").testing.refAllDecls(@This());