Add native El NLG system: morphology, vocabulary, grammar, realizer

Implements a complete natural language generation stack in El:
- morphology.el: English pluralization, verb conjugation (40+ irregulars), determiner agreement
- vocabulary.el: inline seed lexicon (~100 entries: pronouns, nouns, verbs, adjectives, etc.)
- grammar.el: CFG rules (S/NP/VP/PP), slot-map driven tree generator, s-expression renderer
- realizer.el: semantic form -> English text with tense/aspect/agreement, do-support for questions
- nlg.el: JSON-driven public API tying all modules together
- tests/run.sh: acceptance corpus runner (6 tests, all passing)

elp/tests/examples/basic-sentence.el

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+// basic-sentence.el - Realize "I see the cat."
+//
+// SemanticForm: assert, agent=I, predicate=see, patient=the cat, present simple.
+
+fn run_test() -> String {
+    let form: [String] = native_list_empty()
+    let form = native_list_append(form, "intent")
+    let form = native_list_append(form, "assert")
+    let form = native_list_append(form, "agent")
+    let form = native_list_append(form, "I")
+    let form = native_list_append(form, "predicate")
+    let form = native_list_append(form, "see")
+    let form = native_list_append(form, "patient")
+    let form = native_list_append(form, "the cat")
+    let form = native_list_append(form, "location")
+    let form = native_list_append(form, "")
+    let form = native_list_append(form, "tense")
+    let form = native_list_append(form, "present")
+    let form = native_list_append(form, "aspect")
+    let form = native_list_append(form, "simple")
+    return realize(form)
+}
+
+println(run_test())

elp/tests/examples/command.el

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+// command.el - Realize "Go home."
+//
+// SemanticForm: command, predicate=go, location=home.
+
+fn run_test() -> String {
+    let form: [String] = native_list_empty()
+    let form = native_list_append(form, "intent")
+    let form = native_list_append(form, "command")
+    let form = native_list_append(form, "agent")
+    let form = native_list_append(form, "")
+    let form = native_list_append(form, "predicate")
+    let form = native_list_append(form, "go")
+    let form = native_list_append(form, "patient")
+    let form = native_list_append(form, "")
+    let form = native_list_append(form, "location")
+    let form = native_list_append(form, "home")
+    let form = native_list_append(form, "tense")
+    let form = native_list_append(form, "present")
+    let form = native_list_append(form, "aspect")
+    let form = native_list_append(form, "simple")
+    return realize(form)
+}
+
+println(run_test())

elp/tests/examples/past-tense.el

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+// past-tense.el - Realize "She ran in the park."
+//
+// SemanticForm: assert, agent=she, predicate=run, location=in the park, past simple.
+
+fn run_test() -> String {
+    let form: [String] = native_list_empty()
+    let form = native_list_append(form, "intent")
+    let form = native_list_append(form, "assert")
+    let form = native_list_append(form, "agent")
+    let form = native_list_append(form, "she")
+    let form = native_list_append(form, "predicate")
+    let form = native_list_append(form, "run")
+    let form = native_list_append(form, "patient")
+    let form = native_list_append(form, "")
+    let form = native_list_append(form, "location")
+    let form = native_list_append(form, "in the park")
+    let form = native_list_append(form, "tense")
+    let form = native_list_append(form, "past")
+    let form = native_list_append(form, "aspect")
+    let form = native_list_append(form, "simple")
+    return realize(form)
+}
+
+println(run_test())

elp/tests/examples/plural-noun.el

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+// plural-noun.el - Test noun pluralization.
+//
+// Expected: "cats|children|boxes|cities|fish|leaves"
+
+fn run_test() -> String {
+    let r: [String] = native_list_empty()
+    let r = native_list_append(r, pluralize("cat"))
+    let r = native_list_append(r, pluralize("child"))
+    let r = native_list_append(r, pluralize("box"))
+    let r = native_list_append(r, pluralize("city"))
+    let r = native_list_append(r, pluralize("fish"))
+    let r = native_list_append(r, pluralize("leaf"))
+    return str_join(r, "|")
+}
+
+println(run_test())

elp/tests/examples/question.el

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+// question.el - Realize "Do you see the dog?"
+//
+// SemanticForm: question, agent=you, predicate=see, patient=the dog, present simple.
+
+fn run_test() -> String {
+    let form: [String] = native_list_empty()
+    let form = native_list_append(form, "intent")
+    let form = native_list_append(form, "question")
+    let form = native_list_append(form, "agent")
+    let form = native_list_append(form, "you")
+    let form = native_list_append(form, "predicate")
+    let form = native_list_append(form, "see")
+    let form = native_list_append(form, "patient")
+    let form = native_list_append(form, "the dog")
+    let form = native_list_append(form, "location")
+    let form = native_list_append(form, "")
+    let form = native_list_append(form, "tense")
+    let form = native_list_append(form, "present")
+    let form = native_list_append(form, "aspect")
+    let form = native_list_append(form, "simple")
+    return realize(form)
+}
+
+println(run_test())

elp/tests/examples/verb-conjugation.el

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+// verb-conjugation.el - Test verb conjugation.
+//
+// Expected: "ran|walked|is|am|sleeping"
+
+fn run_test() -> String {
+    let r: [String] = native_list_empty()
+    let r = native_list_append(r, verb_form("run",  "past",    "third",  "singular"))
+    let r = native_list_append(r, verb_form("walk", "past",    "third",  "singular"))
+    let r = native_list_append(r, verb_form("be",   "present", "third",  "singular"))
+    let r = native_list_append(r, verb_form("be",   "present", "first",  "singular"))
+    let r = native_list_append(r, verb_form("sleep","progressive","third","singular"))
+    return str_join(r, "|")
+}
+
+println(run_test())