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).
Adds EL_TRUE/EL_FALSE convenience macros to el_runtime.h alongside the
existing EL_NULL, making boolean-returning builtins readable without
raw (el_val_t) casts. Documents all value macros in the header comment.
Also lands el_arena_push/el_arena_pop — a scoped string arena for CLI
programs that never call el_request_start/end. The compiler can push a
mark before a compilation unit and pop it after to free intermediate
strings, reducing peak RSS during long compile runs.
Will Anderson