ffi 1.17: #jni_call(name, sig) literal-keyed slot interning

Two `#jni_call` sites with the same string-literal `(name, sig)` pair
now share a single `jclass` GlobalRef slot and a single `jmethodID`
slot, populated lazily on the first call to any matching site.
Non-literal sites keep the per-call `GetObjectClass` + `GetMethodID`
sequence from step 1.15.

Per-call-site lowering for literal sites:

  %cached_mid = load ptr, @SX_JNI_MID_<key>
  %is_cached  = icmp ne ptr %cached_mid, null
  br i1 %is_cached, cont, miss
miss:
  %local_cls  = GetObjectClass(env, target)
  %global_cls = NewGlobalRef(env, local_cls)     ; vtable slot 21
  store ptr %global_cls, @SX_JNI_CLS_<key>
  %fresh_mid  = GetMethodID(env, global_cls, name, sig)
  store ptr %fresh_mid, @SX_JNI_MID_<key>
  br cont
cont:
  %mid = phi ptr [%cached_mid, before], [%fresh_mid, miss]
  call <Type>Method(env, target, %mid, args...)

Wiring:
- `JniMsgSend.cache_key: ?CacheKey` (new) carries `(name_str,
  sig_str)` when both `name` and `sig` are string-literal AST nodes;
  empty for non-literal call sites.
- `lower.zig` populates `cache_key` from the AST.
- `emit_llvm.zig` `getOrCreateJniSlots(name, sig)` returns the
  `{cls_slot, mid_slot}` pair, creating and caching them on first
  lookup. Key is `name\x00sig` so the separator can't collide with
  any JNI identifier byte.
- `mangleJniKey` builds an LLVM-identifier suffix from the pair, used
  in the `@SX_JNI_{CLS,MID}_<suffix>` global names.

IR snapshot at `tests/expected/ffi-jni-call-03-methodid-sharing.ir`
updated: two call sites against literal `("noop", "()V")` now share
`@SX_JNI_CLS_noop____V` and `@SX_JNI_MID_noop____V`. Pre-1.17 snapshot
had two independent `GetMethodID` calls; post-1.17 has one global
slot pair plus per-call lazy-init branches.

Note: an unrelated regression in `examples/ffi-objc-call-12-rect-u64-returns.sx`
exists in the working tree (parse error from an in-progress C-import
block) and is left untouched.

tests/expected/ffi-jni-call-03-methodid-sharing.ir

@@ -3,6 +3,8 @@
 @g_should_call = internal global i1 false
 @str = private unnamed_addr constant [5 x i8] c"noop\00", align 1
 @str.1 = private unnamed_addr constant [4 x i8] c"()V\00", align 1
+@SX_JNI_CLS_noop____V = internal global ptr null
+@SX_JNI_MID_noop____V = internal global ptr null
 @str.2 = private unnamed_addr constant [5 x i8] c"noop\00", align 1
 @str.3 = private unnamed_addr constant [4 x i8] c"()V\00", align 1
 @str.4 = private unnamed_addr constant [4 x i8] c"ok\0A\00", align 1
@@ -208,27 +210,55 @@ entry:
   %load = load ptr, ptr %alloca, align 8
   %loadN = load ptr, ptr %allocaN, align 8
   %jni.ifs = load ptr, ptr %load, align 8
+  %jni.cached.mid = load ptr, ptr @SX_JNI_MID_noop____V, align 8
+  %jni.is.cached = icmp ne ptr %jni.cached.mid, null
+  br i1 %jni.is.cached, label %jni.cont, label %jni.miss
+
+jni.miss:                                         ; preds = %entry
   %2 = getelementptr inbounds ptr, ptr %jni.ifs, i32 31
   %jni.GetObjectClass = load ptr, ptr %2, align 8
   %jni.cls = call ptr %jni.GetObjectClass(ptr %load, ptr %loadN)
-  %3 = getelementptr inbounds ptr, ptr %jni.ifs, i32 33
-  %jni.GetMethodID = load ptr, ptr %3, align 8
-  %jni.mid = call ptr %jni.GetMethodID(ptr %load, ptr %jni.cls, ptr @str, ptr @str.1)
-  %4 = getelementptr inbounds ptr, ptr %jni.ifs, i32 61
-  %jni.callfn = load ptr, ptr %4, align 8
+  %3 = getelementptr inbounds ptr, ptr %jni.ifs, i32 21
+  %jni.NewGlobalRef = load ptr, ptr %3, align 8
+  %jni.global.cls = call ptr %jni.NewGlobalRef(ptr %load, ptr %jni.cls)
+  store ptr %jni.global.cls, ptr @SX_JNI_CLS_noop____V, align 8
+  %4 = getelementptr inbounds ptr, ptr %jni.ifs, i32 33
+  %jni.GetMethodID = load ptr, ptr %4, align 8
+  %jni.fresh.mid = call ptr %jni.GetMethodID(ptr %load, ptr %jni.global.cls, ptr @str, ptr @str.1)
+  store ptr %jni.fresh.mid, ptr @SX_JNI_MID_noop____V, align 8
+  br label %jni.cont
+
+jni.cont:                                         ; preds = %jni.miss, %entry
+  %jni.mid = phi ptr [ %jni.cached.mid, %entry ], [ %jni.fresh.mid, %jni.miss ]
+  %5 = getelementptr inbounds ptr, ptr %jni.ifs, i32 61
+  %jni.callfn = load ptr, ptr %5, align 8
   call void %jni.callfn(ptr %load, ptr %loadN, ptr %jni.mid)
   %loadN = load ptr, ptr %alloca, align 8
   %loadN = load ptr, ptr %allocaN, align 8
   %jni.ifs5 = load ptr, ptr %loadN, align 8
-  %5 = getelementptr inbounds ptr, ptr %jni.ifs5, i32 31
-  %jni.GetObjectClass6 = load ptr, ptr %5, align 8
-  %jni.cls7 = call ptr %jni.GetObjectClass6(ptr %loadN, ptr %loadN)
-  %6 = getelementptr inbounds ptr, ptr %jni.ifs5, i32 33
-  %jni.GetMethodID8 = load ptr, ptr %6, align 8
-  %jni.mid9 = call ptr %jni.GetMethodID8(ptr %loadN, ptr %jni.cls7, ptr @str.2, ptr @str.3)
-  %7 = getelementptr inbounds ptr, ptr %jni.ifs5, i32 61
-  %jni.callfn10 = load ptr, ptr %7, align 8
-  call void %jni.callfn10(ptr %loadN, ptr %loadN, ptr %jni.mid9)
+  %jni.cached.mid6 = load ptr, ptr @SX_JNI_MID_noop____V, align 8
+  %jni.is.cached7 = icmp ne ptr %jni.cached.mid6, null
+  br i1 %jni.is.cached7, label %jni.cont9, label %jni.miss8
+
+jni.miss8:                                        ; preds = %jni.cont
+  %6 = getelementptr inbounds ptr, ptr %jni.ifs5, i32 31
+  %jni.GetObjectClass10 = load ptr, ptr %6, align 8
+  %jni.cls11 = call ptr %jni.GetObjectClass10(ptr %loadN, ptr %loadN)
+  %7 = getelementptr inbounds ptr, ptr %jni.ifs5, i32 21
+  %jni.NewGlobalRef12 = load ptr, ptr %7, align 8
+  %jni.global.cls13 = call ptr %jni.NewGlobalRef12(ptr %loadN, ptr %jni.cls11)
+  store ptr %jni.global.cls13, ptr @SX_JNI_CLS_noop____V, align 8
+  %8 = getelementptr inbounds ptr, ptr %jni.ifs5, i32 33
+  %jni.GetMethodID14 = load ptr, ptr %8, align 8
+  %jni.fresh.mid15 = call ptr %jni.GetMethodID14(ptr %loadN, ptr %jni.global.cls13, ptr @str.2, ptr @str.3)
+  store ptr %jni.fresh.mid15, ptr @SX_JNI_MID_noop____V, align 8
+  br label %jni.cont9
+
+jni.cont9:                                        ; preds = %jni.miss8, %jni.cont
+  %jni.mid16 = phi ptr [ %jni.cached.mid6, %jni.cont ], [ %jni.fresh.mid15, %jni.miss8 ]
+  %9 = getelementptr inbounds ptr, ptr %jni.ifs5, i32 61
+  %jni.callfn17 = load ptr, ptr %9, align 8
+  call void %jni.callfn17(ptr %loadN, ptr %loadN, ptr %jni.mid16)
   ret void
 }