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.

elp/tests/run.sh

@@ -33,6 +33,7 @@ NLG_MODULES=(
     "${SRC_DIR}/vocabulary.el"
     "${SRC_DIR}/grammar.el"
     "${SRC_DIR}/realizer.el"
+    "${SRC_DIR}/semantics.el"
     "${SRC_DIR}/nlg.el"
 )
 
@@ -102,6 +103,10 @@ run_nlg_case "question"          examples/question.el          "Do you see the d
 run_nlg_case "command"           examples/command.el           "Go home."
 run_nlg_case "plural-noun"       examples/plural-noun.el       "cats|children|boxes|cities|fish|leaves"
 run_nlg_case "verb-conjugation"  examples/verb-conjugation.el  "ran|walked|is|am|sleeping"
+run_nlg_case "sem-assert"        examples/sem-assert.el        "She sleeps."
+run_nlg_case "sem-query"         examples/sem-query.el         "Do you see?"
+run_nlg_case "sem-describe"      examples/sem-describe.el      "The cat is big."
+run_nlg_case "sem-greet"         examples/sem-greet.el         "Hello, Alice."
 
 echo
 echo "${PASS} passed, ${FAIL} failed"