rt: move slice append helpers from lib/slices/ into libwwrt.a

rt/append.ww

@@ -0,0 +1,49 @@
+// rt/append.ww — slice runtime helpers, archived into libwwrt.a.
+//
+// The compiler lowers `append(s, v)` to a CALL to one of these by
+// element size (1 byte → appendu8, else → appendi64). User code
+// never `use`s this — the symbol comes in via the runtime archive,
+// like rt_alloc and rt_streq.
+
+@symbol("rt_alloc") fn alloc(n: u64) *void;
+@symbol("rt_free")  fn free(p: *void, n: u64) void;
+
+export fn appendu8(s: *[]u8, v: u8) void = {
+	if (s.len >= s.cap) {
+		let nc: i32 = s.cap * 2;
+		if (nc < 8) { nc = 8; };
+		let np: *u8 = alloc(nc: u64): *u8;
+		let i: i32 = 0;
+		for (i < s.len) {
+			np[i] = s.ptr[i];
+			i += 1;
+		};
+		if (s.cap > 0) {
+			free(s.ptr: *void, s.cap: u64);
+		};
+		s.ptr = np;
+		s.cap = nc;
+	};
+	s.ptr[s.len] = v;
+	s.len += 1;
+};
+
+export fn appendi64(s: *[]i64, v: i64) void = {
+	if (s.len >= s.cap) {
+		let nc: i32 = s.cap * 2;
+		if (nc < 8) { nc = 8; };
+		let np: *i64 = alloc((nc * 8): u64): *i64;
+		let i: i32 = 0;
+		for (i < s.len) {
+			np[i] = s.ptr[i];
+			i += 1;
+		};
+		if (s.cap > 0) {
+			free(s.ptr: *void, (s.cap * 8): u64);
+		};
+		s.ptr = np;
+		s.cap = nc;
+	};
+	s.ptr[s.len] = v;
+	s.len += 1;
+};