starkernel: item 4.3.3a -- Q48.16 trigonometry (Q.SIN/Q.COS)

Adds q48_reduce_angle() (range-reduce a signed Q48.16 angle into
[-PI_Q48, PI_Q48] via one integer division plus a bounded fix-up loop) and
q48_sin_approx/q48_cos_approx (Taylor series, terms n=3,5,7,9,11 for sin
and n=2,4,6,8,10 for cos, early exit below 10). Q.SIN/Q.COS registered as
FORTH words in q48_words.c, same pattern as Q.LOG/Q.EXP/Q.SQRT.

Found mid-implementation: this codebase has two independent Q48.16
implementations -- src/word_source/q48_16_words.c (hosted/vendored) and
src/starkernel/math/q48_16.c (kernel-only; the kernel build does not
compile the former at all). The hosted build linked fine after the first
pass; the kernel build failed with undefined references until the same
two functions were added to both .c files and both q48_16.h headers
(include/q48_16.h and include/starkernel/q48_16.h). Not fixed at the root
-- Q.LOG/Q.EXP/Q.SQRT already had this same four-file duplication,
unremarked until now -- just navigated correctly for this item.

Verified live on amd64 via serial injection: sin/cos at 0, +-pi/2, pi, and
3pi (range-reduction across multiple turns) all match expected values
within Taylor-series truncation error (<0.2%).

All three architectures boot clean to ok> with the DoE completing;
dict_hash identical across all three (0x291a660b05fa7b52).

FABRIC.md item 4.3.3a marked done with full acceptance evidence.

src/word_source/q48_words.c

@@ -24,6 +24,8 @@
  *   Q.LOG   ( q -- ln_q )         natural log, Newton-Raphson
  *   Q.EXP   ( q -- e^q )          Taylor series
  *   Q.SQRT  ( q -- sqrt_q )       Newton-Raphson
+ *   Q.SIN   ( q -- sin_q )        Taylor series, radians, any magnitude
+ *   Q.COS   ( q -- cos_q )        Taylor series, radians, any magnitude
  *   Q.FROM-INT ( n -- q )         integer → Q48.16 (n << 16)
  *   Q.TO-INT   ( q -- n )         Q48.16 → integer (q >> 16, truncate)
  *   Q.1     ( -- 65536 )          1.0 in Q48.16
@@ -115,6 +117,16 @@ static void q48_word_sqrt(VM *vm)
     q48_push(vm, q48_sqrt_approx(q48_pop(vm)));
 }
 
+static void q48_word_sin(VM *vm)
+{
+    q48_push(vm, q48_sin_approx(q48_pop(vm)));
+}
+
+static void q48_word_cos(VM *vm)
+{
+    q48_push(vm, q48_cos_approx(q48_pop(vm)));
+}
+
 /* ── conversions ─────────────────────────────────────────────────────── */
 
 static void q48_word_from_int(VM *vm)
@@ -208,6 +220,8 @@ void register_q48_words(VM *vm)
     register_word(vm, "Q.LOG",      q48_word_log);
     register_word(vm, "Q.EXP",      q48_word_exp);
     register_word(vm, "Q.SQRT",     q48_word_sqrt);
+    register_word(vm, "Q.SIN",      q48_word_sin);
+    register_word(vm, "Q.COS",      q48_word_cos);
     register_word(vm, "Q.FROM-INT", q48_word_from_int);
     register_word(vm, "Q.TO-INT",   q48_word_to_int);
     register_word(vm, "Q.1",        q48_word_one);