runtime: math_log is base-10, not natural log

el_val_t math_log(el_val_t f) { return el_from_float(log(el_to_float(f))); }
    el_val_t math_ln(el_val_t f)  { return el_from_float(log(el_to_float(f))); }

Both were natural log, so math_log and math_ln were the same function.
log10(100) returned 4.605 instead of 2.

Three sources already agreed it should be base-10 and were being contradicted
by this one line:
  - runtime/math.el:55  "// math_log — base-10 logarithm."
  - el_seed.c:1278      __log_f -> log10()  (the path math.el actually calls)
  - tests/native/test_math.el:133  asserts log10(100) == 2

FOUND BY THE NEW TEST FRAMEWORK ON ITS FIRST RUN (el #133). The assertion had
been sitting in the suite the whole time; nothing could report it. The old
harness printed "N passed, M failed" with no per-test detail, and half the
suites were not compiling at all — so a failing assertion in a suite nobody
could run was indistinguishable from no failure.

That is the entire argument for the framework, demonstrated on day one: this is
not a bug the framework introduced, it is a bug the framework made VISIBLE.

Verified: tests/native/test_math.el goes 12/13 -> 13/13, math-log passing.

lang/runtime/el_runtime.c

@@ -5240,7 +5240,12 @@ el_val_t str_to_float(el_val_t s) {
 /* ── Math (Float-aware) ──────────────────────────────────────────────────── */
 
 el_val_t math_sqrt(el_val_t f) { return el_from_float(sqrt(el_to_float(f))); }
-el_val_t math_log(el_val_t f)  { return el_from_float(log(el_to_float(f))); }
+/* base-10, matching runtime/math.el's documented contract ("math_log — base-10
+ * logarithm") and el_seed.c's __log_f. This returned NATURAL log, so math_log
+ * and math_ln were the same function: log10(100) gave 4.605 instead of 2.
+ * Caught by tests/native/test_math.el on the new framework's first run — the
+ * assertion existed all along, the suite just had no way to report it. */
+el_val_t math_log(el_val_t f)  { return el_from_float(log10(el_to_float(f))); }
 el_val_t math_ln(el_val_t f)   { return el_from_float(log(el_to_float(f))); }
 el_val_t math_sin(el_val_t f)  { return el_from_float(sin(el_to_float(f))); }
 el_val_t math_cos(el_val_t f)  { return el_from_float(cos(el_to_float(f))); }