Add --test mode to elc with Assert stmt and full native test suite passing

Implement compile_test() entry point that emits a C test harness instead
of a normal program. Test blocks (previously skipped) now compile to
static functions with per-assertion pass/fail tracking. Assert statement
added to parser and codegen. Runtime extended with now_ns, fs_list_json,
json_build_object, json_build_array, json_escape_string, state_has,
state_get_or. Fix float negation codegen, float equality comparisons,
time_to_parts return type (JSON string), time_format empty-fmt, json_set
raw-value semantics, state_keys JSON array return. All 310 native tests
pass across 9 suites (core, text, string, math, env, state, json, time, fs).

lang/el-compiler/src/parser.el

@@ -1684,6 +1684,28 @@ fn parse_stmt(tokens: [Any], pos: Int) -> Map<String, Any> {
         }, p)
     }
 
+    // assert <cond_expr> [ , <msg_expr> ]
+    // The message is optional — if the next token after the condition is not a
+    // Comma, emit an empty string placeholder so the test still works.
+    if k == "Assert" {
+        let p: Int = pos + 1
+        let cond_r = parse_expr(tokens, p)
+        let cond_node = cond_r["node"]
+        let p: Int = cond_r["pos"]
+        el_release(cond_r)
+        let after_k: String = tok_kind(tokens, p)
+        if str_eq(after_k, "Comma") {
+            let p = p + 1
+            let msg_r = parse_expr(tokens, p)
+            let msg_node = msg_r["node"]
+            let p: Int = msg_r["pos"]
+            el_release(msg_r)
+            return make_result({ "stmt": "Assert", "cond": cond_node, "msg": msg_node }, p)
+        }
+        // No message — use empty string placeholder.
+        return make_result({ "stmt": "Assert", "cond": cond_node, "msg": { "expr": "Str", "value": "" } }, p)
+    }
+
     // Bare reassignment: `name = expr`. Handled BEFORE the expression
     // fallback so we don't drop the assign on the floor and emit three
     // orphan expressions (the original silent-miscompile bug). El's `let`