ffi 2.16c green: TL fallback via C-helper runtime + always-omit env in #jni_call

`#jni_call` collapses to a single surface — env is *always* implicit:
either picked up from the lexically-enclosing `#jni_env(env) { ... }`
block's Ref (cheap, register-resident, no TL touch) or from the
runtime's thread-local slot via `sx_jni_env_tl_get()` (one fn call
per dispatch). The explicit-env shape is gone — chess and the
existing tests migrate cleanly by wrapping their helper-fn bodies
in `#jni_env(env) { ... }`.

The TL slot lives outside the user's IR module so the LLVM ORC JIT
can load object files cleanly without `orc_rt` for TLS support:

  library/vendors/sx_jni_runtime/sx_jni_env_tl.c:
    static _Thread_local void *sx_jni_env_tl_slot;
    void *sx_jni_env_tl_get(void) { return sx_jni_env_tl_slot; }
    void sx_jni_env_tl_set(void *env) { sx_jni_env_tl_slot = env; }

Linkage:
- sx-the-compiler links the .c file via build.zig so the JIT
  process-symbol generator resolves `sx_jni_env_tl_get`/`_set`.
- AOT targets get the same .c file auto-linked via the lowering
  pass: when lower touches the TL externs, it sets
  `needs_jni_env_tl_runtime`, and `Compilation.lowerToIR` appends a
  synthetic `CImportInfo` to `lowering_extra_c_sources` that
  `collectCImportSources` merges with user-written ones.

Lowering-side changes:
- `getJniEnvTlFids` lazily declares the two externs (parallel
  to `getSelRegisterNameFid`) and flips `needs_jni_env_tl_runtime`.
- `#jni_env(env) { body }` emits save→set→body→restore via three
  `call` ops to the externs; the inner body sees env via the
  lexical-direct stack.
- `lowerJniCall` resolves env from `jni_env_stack` (top) or the TL
  fallback. The explicit-env branch is gone.
- `jni_env_stack_base` tracks per-fn lexical scope so lazy-lowering
  a callee doesn't accidentally see the caller's Ref (Refs are only
  valid inside one fn's instruction stream).

Test migration (mechanical):
- ffi-jni-call-{01..09}: each helper fn wraps `#jni_call(...)`
  bodies in `#jni_env(env) { ... }`. Returning values pass through
  the block as an expression — `#jni_env` now also lowers in
  expression position.

Verified:
- zig build test + tests/run_examples.sh: 130/130 green.
- tests/cross_compile.sh: 3/3 green.
- Chess APK rebuilt + reinstalled on Pixel. Board renders with
  status-bar clearance + info panel intact; no crashes in logcat.
  Safe-insets dispatch through `#jni_env` + lexical-direct now
  fully exercised end-to-end on real hardware.

src/core.zig

@@ -28,6 +28,10 @@ pub const Compilation = struct {
     import_graph: std.StringHashMap(std.StringHashMap(void)),
     sema_result: ?sema.SemaResult = null,
     ir_emitter: ?ir.LLVMEmitter = null,
+    /// C sources requested by the lowering pass (not in the user's AST).
+    /// E.g. the JNI env TL runtime when `#jni_env` is used. Merged with
+    /// AST sources in `collectCImportSources`.
+    lowering_extra_c_sources: std.ArrayList(c_import.CImportInfo) = .empty,
 
     pub fn init(allocator: std.mem.Allocator, io: std.Io, file_path: []const u8, source: [:0]const u8, target_config: TargetConfig, stdlib_paths: []const []const u8) Compilation {
         return .{
@@ -145,10 +149,36 @@ pub const Compilation = struct {
         return null;
     }
 
-    /// Collect C import source info from the resolved AST.
+    /// Collect C import source info — both from user-written `#import c { ... }`
+    /// blocks in the AST AND from lowering-time auto-injections (currently:
+    /// the JNI env TL runtime when `#jni_env` / `#jni_call`-with-omitted-env
+    /// is used). The lower-side auto-injections live in
+    /// `lowering_extra_c_sources` and are populated by `lowerToIR` based on
+    /// `Lowering.needs_jni_env_tl_runtime` etc.
     pub fn collectCImportSources(self: *Compilation) ![]c_import.CImportInfo {
         const root = self.resolved_root orelse self.root orelse return &.{};
-        return c_import.collectCImportSources(self.allocator, root);
+        const ast_sources = try c_import.collectCImportSources(self.allocator, root);
+        if (self.lowering_extra_c_sources.items.len == 0) return ast_sources;
+        var merged = std.ArrayList(c_import.CImportInfo).empty;
+        try merged.appendSlice(self.allocator, ast_sources);
+        try merged.appendSlice(self.allocator, self.lowering_extra_c_sources.items);
+        return merged.toOwnedSlice(self.allocator);
+    }
+
+    /// Resolve a stdlib-relative path through the configured `stdlib_paths`.
+    /// Returns the first candidate whose absolute path resolves to an
+    /// existing file. Used by lower-side auto-injected C sources.
+    fn resolveStdlibPath(self: *Compilation, rel: []const u8) !?[]const u8 {
+        for (self.stdlib_paths) |root_path| {
+            const candidate = try std.fmt.allocPrint(self.allocator, "{s}/{s}", .{ root_path, rel });
+            if (std.Io.Dir.readFileAlloc(.cwd(), self.io, candidate, self.allocator, .limited(1024 * 1024))) |buf| {
+                self.allocator.free(buf);
+                return candidate;
+            } else |_| {
+                self.allocator.free(candidate);
+            }
+        }
+        return null;
     }
 
     /// Lower the parsed AST to the sx IR module (shadow pipeline).
@@ -168,6 +198,23 @@ pub const Compilation = struct {
         lowering.import_graph = &self.import_graph;
         lowering.lowerRoot(root);
         if (self.diagnostics.hasErrors()) return error.CompileError;
+
+        // Auto-link the JNI env TL runtime when lowering used it. The .c file
+        // ships with the sx library; we resolve it through stdlib_paths so
+        // consumers don't need to vendor a copy.
+        if (lowering.needs_jni_env_tl_runtime) {
+            if (try self.resolveStdlibPath("vendors/sx_jni_runtime/sx_jni_env_tl.c")) |abs_path| {
+                var sources = std.ArrayList([]const u8).empty;
+                try sources.append(self.allocator, abs_path);
+                try self.lowering_extra_c_sources.append(self.allocator, .{
+                    .sources = try sources.toOwnedSlice(self.allocator),
+                    .includes = &.{},
+                    .defines = &.{},
+                    .flags = &.{},
+                });
+            }
+        }
+
         return module;
     }