wcc/cgen: #145 slice-copy-assign LHS s.arr[lo:hi]=bs — N_SLICE-LHS arm, runtime byte-copy loop, esz via type table (both stages)

Probe-first find for the path c2 appendlit (buf.buf[lo..hi]=bs): a
slice-copy-assign into a struct-field array sub-range emitted ZERO code —
silent NO-OP, both stages, both-wrong-identical (#263), so runtime is the
only net. N_ASSIGN gains an N_SLICE-LHS arm (cgen.c + cgenexpr.ww
slicebaseesz twin) reusing the N_SLICE-read base/esz cascade and copying
(hi-lo)*esz bytes from rhs.ptr via a runtime loop (len is runtime; no
REP/MOVSB). esz routed through the type table (rule 13; [N]u8->1). Hare
len(bs)==hi-lo assert deferred to #149.

selfhost/cmd/wcc/cgenexpr.ww

@@ -2336,6 +2336,39 @@ fn cgbasecap(c: *cgen, base: *node, dst: str) bool = {
 // remaining to the base's end (#20, Go/Hare-identical) via cgbasecap.
 // ptr advances by BYTES (lo*esz, #76; ref/hare/rt/ensure.ha:30
 // membsz-unit); esz from the type table, mirroring the cgindex idiom.
+// slicebaseesz — element width of a sliceable base, exactly mirroring the
+// esz cascade cgslice computes inline for ptr-scaling (baselocal/globaltn →
+// elemsizeofc; N_DOT field → dotbu.sub.size; N_ARRLIT → elemsizeofc; alias-
+// NAMED override → chased sub.size). The #145 slice-copy-assign arm scales
+// the COUNT by this same width, so it MUST match cgslice's ptr scaling
+// byte-for-byte (cgslice supplies the dst ptr). Cstage twin: the N_SLICE-LHS
+// arm in cgen.c N_ASSIGN reuses the read-path one-liner directly.
+fn slicebaseesz(c: *cgen, base: *node) i32 = {
+	if (base == nil) { return 1; };
+	let esz: i32 = 1;
+	if (base.kind == nkind.N_IDENT) {
+		let bl: *local = localfindnode(c, base.str);
+		if (bl != nil) {
+			esz = elemsizeofc(c, bl.tnode);
+		} else {
+			let gt: *node = letvartnode(c, base.str);
+			if (gt != nil) { esz = elemsizeofc(c, gt); };
+		};
+		let bt: *tinfo = base.type_: *tinfo;
+		if (bt != nil) { if (bt.kind == tykind.TY_NAMED) {
+			let bu60: *tinfo = tichase(bt);
+			let es60: *tinfo = tichase(bu60.sub);
+			if (es60 != nil) { esz = es60.size: i32; };
+		};};
+	} else { if (base.kind == nkind.N_DOT) {
+		let db: *tinfo = tichase(base.type_: *tinfo);
+		if (db != nil) { if (db.sub != nil) { esz = db.sub.size: i32; }; };
+	} else { if (base.kind == nkind.N_ARRLIT) {
+		esz = elemsizeofc(c, base.lhs);
+	};};};
+	return esz;
+};
+
 fn cgslice(c: *cgen, n: *node) void = {
 	let base: *node = n.lhs;
 	let lo: *node = n.rhs;
@@ -7760,6 +7793,47 @@ fn cgcall(c: *cgen, n: *node) void = {
 
 fn cgassign(c: *cgen, n: *node) void = {
 	let lhs: *node = n.lhs;
+	// #145 (c1.5a): bulk slice-copy-assign `s.arr[lo:hi] = bs` (LHS is
+	// N_SLICE). No legacy cgassign arm catches N_SLICE — the statement
+	// silently emitted nothing (both stages, byte-id-green, #263-class).
+	// Twin of cstage cgen.c N_ASSIGN N_SLICE arm (full WHY there).
+	// cgexpr(lhs) routes to cgslice and leaves AX = dst ptr (base+lo*esz),
+	// BX = hi-lo (element count), CX = cap; slicebaseesz gives the SAME
+	// esz cgslice scaled the ptr by, so BX*esz is the byte count. Then a
+	// runtime-counted byte-granular copy from bs.ptr — byte loop because
+	// the length is RUNTIME (w6a has no REP/MOVSB). Only plain `=`. The
+	// Hare len(bs)==hi-lo assert is task #149 (rule-7: documented, not a
+	// c2 blocker — appendlit's lengths are equal by construction).
+	if (lhs != nil) { if (lhs.kind == nkind.N_SLICE
+	    && n.op == tkind.TK_ASSIGN) {
+		let esz: i32 = slicebaseesz(c, lhs.lhs);
+		cgexpr(c, lhs);
+		if (esz > 1) {
+			emitline("\tMOVQ\t$");
+			emitint(esz: i64);
+			emitline(", DX\n");
+			emitline("\tIMULQ\tDX, BX\n");
+		};
+		emitline("\tPUSHQ\tAX\n");
+		emitline("\tPUSHQ\tBX\n");
+		cgexpr(c, n.rhs);
+		emitline("\tMOVQ\tAX, SI\n");
+		emitline("\tPOPQ\tCX\n");
+		emitline("\tPOPQ\tDI\n");
+		let top: str = mklabel(c, "scpy");
+		let end: str = mklabel(c, "scpe");
+		emitlabel(top);
+		emitline("\tCMPQ\t$0, CX\n");
+		emitline("\tJLE\t"); emitline(end); emitline("\n");
+		emitline("\tMOVB\t(SI), AX\n");
+		emitline("\tMOVB\tAX, (DI)\n");
+		emitline("\tADDQ\t$1, SI\n");
+		emitline("\tADDQ\t$1, DI\n");
+		emitline("\tSUBQ\t$1, CX\n");
+		emitline("\tJMP\t"); emitline(top); emitline("\n");
+		emitlabel(end);
+		return;
+	};};
 	// #20 (task): struct-lit rhs into an INDEXED struct element —
 	// `a[i] = pt{...}`, `(*ts)[i].caps[k] = capture{...}` — a
 	// DEREF place (`*p = pt{...}`) or an indexed-base FIELD place