w6c: slice reassignment — full triple flow through N_IDENT/N_SLICE/N_ASSIGN

N_IDENT for a slice local now loads (AX=ptr, BX=len, CX=cap), matching
the existing global-slice load.

cgexpr learns N_SLICE: `base[lo:hi]` leaves the same triple in
registers, so callers (return, arg push, reassignment) all share
one shape. The let-init's pre-existing N_SLICE direct-store path
stays as a specialisation; the new generic slice let-init catches
fn-returning-slice and slice-ident initialisers.

N_ASSIGN gains a TY_SLICE branch parallel to TY_STR: store all
three halves to the local slot or, for globals, stash CX into DI
before LEAQ-ing the address (CX is both the new cap and the
address scratch).

test/wcc/630_let_global.c

@@ -146,11 +146,10 @@ static const struct fixture fixtures[] = {
 	},
 	{
 		"slice-cap-via-raw-pointer",
-		/* Drive the slice header through a raw u64 pointer cast
-		 * — the language has no slice-reassignment expression
-		 * yet, so this is the only way to fill the header from
-		 * ww source today. .cap reads back as the value we
-		 * wrote. */
+		/* Drive the slice header through a raw u64 pointer cast.
+		 * Pre-fix for slice reassignment this was the only way to
+		 * fill the header from ww source; kept because the path
+		 * still has to work. */
 		"let buf: []u8;\n"
 		"fn main() i32 = {\n"
 		"\tlet p: *u64 = (&buf): *u64;\n"
@@ -159,6 +158,52 @@ static const struct fixture fixtures[] = {
 		"};\n",
 		99,
 	},
+	{
+		"slice-global-reassign-from-global",
+		/* `dst = src` for slice globals — both halves of the
+		 * triple (ptr, len, cap) must propagate. */
+		"let dst: []u8;\n"
+		"let src: []u8;\n"
+		"fn main() i32 = {\n"
+		"\tlet p: *u64 = (&src): *u64;\n"
+		"\tp[0] = 0x1000u64;\n"
+		"\tp[1] = 7u64;\n"
+		"\tp[2] = 99u64;\n"
+		"\tdst = src;\n"
+		"\treturn dst.len: i32 + dst.cap: i32;\n"
+		"};\n",
+		106,
+	},
+	{
+		"slice-local-reassign-from-slice-expr",
+		/* `s = arr[lo:hi]` reassigns the slice local; cgexpr now
+		 * emits N_SLICE as a triple. Previously the let-init
+		 * specific N_SLICE path worked but reassignment dropped
+		 * len/cap. */
+		"fn main() i32 = {\n"
+		"\tlet arr: [16]u8;\n"
+		"\tlet i: i32 = 0;\n"
+		"\tfor (i < 16) { arr[i] = i: u8; i += 1; };\n"
+		"\tlet s: []u8 = arr[3:10];\n"
+		"\ts = arr[1:5];\n"
+		"\treturn s.len: i32;\n"
+		"};\n",
+		4,
+	},
+	{
+		"slice-local-from-fn-return",
+		/* fn returning []u8 leaves (AX,BX,CX) at return; the let-
+		 * init slice fallback stores the triple. */
+		"fn mkslice() []u8 = {\n"
+		"\tlet arr: [16]u8;\n"
+		"\treturn arr[3:9];\n"
+		"};\n"
+		"fn main() i32 = {\n"
+		"\tlet s: []u8 = mkslice();\n"
+		"\treturn s.len: i32;\n"
+		"};\n",
+		6,
+	},
 	{
 		"struct-field-readwrite",
 		/* Top-level struct global. Field writes hit the global