ffi 2.16a green: parser + AST + sema for #jni_env(env) { body }

New `hash_jni_env` lexer token; `parsePrimary` dispatches to a small
`parseJniEnvBlock` that consumes `(env) { body }` and returns a new
`JniEnvBlock` AST node (env_expr + body block).

Sema's analyzeNode arm recurses into env + body inside a pushed
scope; findNodeAtOffset descends through both children for go-to-
definition.

Lowering treats it as a syntactic wrapper around the block: env is
evaluated for side effects, body lowers as a normal block. The TL
push/pop semantics (synthesizing the env stack so `#jni_call`'s env
arg can become optional) land in 2.16b.

`expectSemicolonAfter` recognises `jni_env_block` as block-form so
statement-position uses don't need a trailing `;` — matches `if` /
`while` / `for` / bare blocks.

Test runs through the block body and prints expected output; xfail
snapshot flips to green. 127/127 examples green.

src/ir/lower.zig

@@ -1050,6 +1050,13 @@ pub const Lowering = struct {
             .destructure_decl => |dd| self.lowerDestructureDecl(&dd),
             .insert_expr => |ins| self.lowerInsertExpr(ins.expr),
             .block => self.lowerBlock(node),
+            .jni_env_block => |eb| {
+                // 2.16a: evaluate env for side effects, lower body as a normal block.
+                // TL push/pop semantics land in 2.16b; until then `#jni_env` is a
+                // syntactic marker that doesn't affect codegen.
+                _ = self.lowerExpr(eb.env);
+                self.lowerBlock(eb.body);
+            },
             // Block-local type declarations
             .struct_decl => |sd| self.registerStructDecl(&sd),
             .enum_decl, .union_decl => {