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.

selfhost/cmd/wcc/cgenexpr.ww

@@ -642,6 +642,22 @@ fn cgident(c: *cgen, n: *node) void = {
 				return;
 			};
 		};
+		// Float def: load via LEAQ + MOVSS/MOVSD into X0, same shape
+		// as the let-float arm below — MOVSS/MOVSD have no D_EXTERN
+		// operand form. Pre-#129 fell through to the MOVQ-AX
+		// integer-convention fallback, leaving X0 untouched (#129
+		// LOAD-side twin of the emitfloatlitdata DATA-side SSoT).
+		if (isfloattype(c, n)) {
+			let mov: str = "MOVSD";
+			if (isf32type(c, n)) { mov = "MOVSS"; };
+			emitline("\tLEAQ\t");
+			emitsymname(c, nm);
+			emitline("(SB), CX\n");
+			emitline("\t");
+			emitline(mov);
+			emitline("\t(CX), X0\n");
+			return;
+		};
 		emitline("\tMOVQ\t");
 		emitsymname(c, nm);
 		emitline("(SB), AX\n");