selfhost: port float lex + expression cgen — feature parity with C

Lexer: `lexnum` now parses the digit/exponent tail into an f64 via a
new `parsef64` (decimal-only, integer-arith driver + pow-10 multiply,
no strtod). The IEEE bits are also stashed in tok.uval via pointer
reinterpret so cgen consumers stay integer-only.

Parser: TK_FLOAT → N_FLOATLIT, carrying both fval and uval. Parser
state grows curfval to plumb the lexer's f64 through refill.

cgen:
  - cgfloatlit reads n.uval and materialises X0 via the standard
    MOVQ-PUSHQ-MOVSD-ADDQ trampoline.
  - cglet, cgident, cgassign learn float-typed branches: MOVSS/MOVSD
    for locals; LEAQ-indirect MOVSS/MOVSD for globals.
  - cgbin handles ADDSD/SUBSD/MULSD/DIVSD (+ SS variants) and
    UCOMISD/UCOMISS-based comparisons. cgun handles float negate
    via the `0 - X0` shape C cgen uses.
  - cgcast routes int↔float and f32↔f64 through CVTSI2SD/CVTTSD2SI/
    CVTSD2SS/CVTSS2SD and their SS twins.
  - cgcall + pushargsrev push float args via SUBQ+MOVSD and pop into
    the X0..X7 stream, tracked by a per-class counter alongside the
    int DI..R9 stream. cgfnparams loads float params from the same
    stream.
  - emitletdataw bakes FLOATLIT init bits into DATAW (4B for f32,
    8B for f64).

Tests: smoke programs (literal init, reassign, arithmetic, fn args/
returns, casts) produce byte-identical asm through `w6c` and
`wwdump_ww -c`, and the resulting binary exits with the same value
whether compiled by the C or wwstage toolchain. Full `make test` is
26/26 and `make bootstrap` still reaches its byte-identical
ww2==ww3==ww4 fixed point.

selfhost/cmd/wcc/cgenstmt.ww

@@ -467,6 +467,19 @@ fn cglet(c: *cgen, n: *node) void = {
 			};
 		};
 		cgexpr(c, rhs);
+		// Float local: cgexpr leaves the value in X0. Spill via
+		// MOVSS (f32, 4B) or MOVSD (f64, 8B).
+		if (isfloattype(c, n.lhs)) {
+			let mov: str = "MOVSD";
+			if (isf32type(c, n.lhs)) { mov = "MOVSS"; };
+			emitline("\t");
+			emitline(mov);
+			emitline("\tX0, ");
+			emitoff(off: i64);
+			emitline("(BP)\n");
+			c.lastwasreturn = 0;
+			return;
+		};
 		emitline("\tMOVQ\tAX, ");
 		emitoff(off: i64);
 		emitline("(BP)\n");