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/cgendecl.ww

@@ -124,9 +124,29 @@ fn scanlocals(c: *cgen, n: *node) i32 = {
 fn cgfnparams(c: *cgen, params: *node) void = {
 	let p: *node = params;
 	let idx: i32 = 0;
+	let fidx: i32 = 0;
 	for (p != nil) {
 		if (p.kind == nkind.N_PARAM) {
 			let nm: str = p.str;
+			if (isfloattype(c, p.lhs)) {
+				// Float param: SysV uses the XMM stream
+				// (X0..X7). 8B (f64) or 4B (f32) slot.
+				let fsz: i32 = 8;
+				if (isf32type(c, p.lhs)) { fsz = 4; };
+				let off: i32 = localadd(c, nm, fsz, p.lhs);
+				let mov: str = "MOVSD";
+				if (fsz == 4) { mov = "MOVSS"; };
+				emitline("\t");
+				emitline(mov);
+				emitline("\t");
+				emitline(fargregname(fidx));
+				emitline(", ");
+				emitoff(off: i64);
+				emitline("(BP)\n");
+				fidx += 1;
+				p = p.next;
+				continue;
+			};
 			if (istaggedtype(p.lhs)) {
 				// tagged-union param: spill size/8 registers
 				// (tag + value words). Slot sized to match.