wcc: float-typed def/let DATA emit via SSoT helper (#129 A.1)

Extract emit_floatlit_data helper for def/let with float-typed top-level
initializer; replaces inline emit_lets float arm and adds previously-
absent emit_defs float arm. Helper peels N_CAST then N_UN(±, N_FLOATLIT),
bit-casts magnitude (f32 via union narrow), emits in little-endian byte
order, applies sign-XOR to top byte inside the loop (bit 31 for f32, bit
63 for f64). The byte-loop XOR avoids materialising 2^63, sidestepping
the strconv.i64tos INT64_MIN bug (#144 / task #37) on the wwstage self-
build path. Mirrored cstage (cgen.c) and wwstage (cgen.ww). Both stages'
float-typed def materialisation (cgexpr N_IDENT / cgident def-branch)
widened to route through the same LEAQ+MOVSS/MOVSD shape as float-typed
let.

Closes 3 latent bugs (all bootstrap-NEUTRAL, no current consumer):
- def: f64 = literal silently emitted undefined ref
- let: f64 = -literal silently emitted undefined ref (N_UN peel absent)
- wwstage let: f32 = literal silently truncated to low 4 of f64 bits

Test 917 (7 rows: f64_def_pos / f64_def_neg / f32_def_pos / f32_def_neg
matrix-closure / f64_let_neg / f64_let_pos / f32_let_pos) registered.
Make test: 180/180 incl. 990-997 byte-id + combined_ww_fresh + 995_self_
rebuild.

#144 (strconv.i64tos INT64_MIN two's-complement-overflow root) filed
separately as task #37 for its own fold.

Makefile

@@ -339,6 +339,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_arr_u16_store_run \
 	  $(BIN)/test_arr_module_index_run \
 	  $(BIN)/test_arr_float_call_index_run \
+	  $(BIN)/test_def_float_lit_run \
 	  $(BIN)/test_f64cgen_run \
 	  $(BIN)/test_f64crossmod_run \
 	  $(BIN)/test_tuprecv_run \
@@ -1147,6 +1148,11 @@ $(BIN)/test_arr_float_call_index_run: test/wcc/916_arr_float_call_index_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_def_float_lit_run: test/wcc/917_def_float_lit_run.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_f64cgen_run: test/wcc/951_f64cgen_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<