Add semantics layer bridging intent frames to grammar realization

Introduces semantics.el with SemFrame (sem_frame/sem_frame_simple/sem_frame_obj
constructors), sem_to_spec to convert intent frames into realizer slot maps,
and sem_realize/sem_realize_full as end-to-end frame→text entry points.
Supports intents: assert, query, describe, greet.

Wires generate_frame() into nlg.el and adds 4 new passing tests
(sem-assert, sem-query, sem-describe, sem-greet). All 10 tests pass.

tests/examples/sem-assert.el

@@ -0,0 +1,10 @@
+// sem-assert.el - Realize "She sleeps." via the semantics layer.
+//
+// SemFrame: intent="assert", subject="she", object="sleep" (verb), modifiers=""
+
+fn run_test() -> String {
+    let frame: [String] = sem_frame("assert", "she", "sleep", "")
+    return sem_realize(frame)
+}
+
+println(run_test())