w6c+w6a+selfhost+lib: cgen+asm bugs surfaced by hash modules

Seven fixes across the toolchain, plus three new lib/hash modules
(adler32, crc16, crc32) that surfaced them.

  1. `~x` on u8/u16/u32 left the upper bits set: NOTQ inverts the
     whole 64-bit register and nothing trimmed it back to type
     width, so a returned `u16` would compare 64-bit against a
     typed literal and disagree. Both stages now mask after NOTQ
     for narrow unsigned: AND $0xFF/0xFFFF for u8/u16, MOVL r,r for
     u32 (ANDQ $0xFFFFFFFF sign-extends imm32 and is a no-op).
     Signed narrows stay sign-extended and need no fix-up. See
     cmd/w6c/cgen.c N_UN TK_TILDE and selfhost cgenexpr.ww cgun
     TK_TILDE with new nodeprimwidth helper.

  2. w6a had no D_CONST immediate path for ANDQ / ORQ. cgen would
     emit `ANDQ $65535, AX` and the rr encoder silently wrote
     `21 /r` with garbage reg fields — the mask never happened.
     Added `81 /4` (AND) and `81 /1` (OR) imm32 paths in both
     cstage and selfhost w6a. The ~width fix above depends on this.

  3. `s: []u8` cast as a direct fn argument produced a 0-length
     slice. cgexpr for N_CAST left (AX=ptr, BX=len) from the str
     source but never set CX (cap), and the arg-push fallback only
     pushed AX. cgcast now synthesises CX=BX when target is slice
     and source is str; node_isslice / arg-push recognise
     cast-to-slice and emit the full (cap, len, ptr) triple. Both
     stages.

  4. `*[N]T` element-store used 8-byte stride + MOVQ regardless of
     T's width. Indexing `buf: *[4]u16` would step 8 bytes and
     write 8 bytes per element. Added idx_eff (drills *[N]T → T)
     in cstage and the matching pointer-array drill in selfhost
     elemsizeof. Also added MOVW / MOVZWQ / MOVSWQ to w6c, w6a,
     and selfhost mirrors so 2-byte element stores/loads use the
     right opcode (was falling through to MOVQ and trailing 6 bytes
     into the next slot).

  5. Slicing a top-level fixed array (`g[0:n]` where `g: [N]T` is
     a global) computed the base from BP instead of the symbol —
     localfind returned 0 and the cgen treated it as a local at
     offset 0. Both N_SLICE-as-expression (cgslice) and N_SLICE-
     as-call-arg paths now check let_islet / letvartnode and emit
     LEAQ name(SB) when the base is a global array (or MOVQ
     name(SB) for a global slice/pointer base). Both stages.

  6. Top-level `let arr: [N]T = [v0, v1, ...]` link-failed on
     cstage — emit_lets bailed when it saw N_ARRLIT init on an
     array type, and the sz==8 scalar path then misemitted any
     8-byte-sized array (e.g. [4]u16, [8]u8) as a single quad.
     emit_lets now walks N_ARRLIT, evaluates each element as an
     int/rune/bool/nil literal, packs per-element bytes
     little-endian, and honours the trailing `...` repeat marker.
     Selfhost already handled the literal-init path; fixed the
     parallel sz==8 duplicate-DATAW emit on its side (the array
     and the scalar paths both fired, last write winning at link
     but the duplicate broke cross-stage byte-identicality on user
     code with this shape).

  7. w6a's per-line input buffer was a 1KB stack `char buf[1024]`.
     A `DATAW` for a [256]u16 emits ~2080 bytes on one line, which
     truncated mid-escape; the assembler then re-parsed the
     remaining tail as garbage opcodes ("unknown opcode"). Bumped
     cstage w6a to a 32K static buffer (selfhost w6a already
     allocated per-line via amalloc).

  lib: lib/hash/adler32, lib/hash/crc16, lib/hash/crc32 — pure
  buffer-subset shape (matching lib/hash/fnv), with per-module
  *_test.ww runnable via `ww test lib/hash/<name>`. Adler-32 plus
  CRC-16 (CCITT/CMDA2000/DECT/ANSI) and CRC-32 (IEEE/Castagnoli/
  Koopman) cover Hare's reference vectors bit-for-bit. Wired into
  test/wcc/900_stdlib.c. .gitignore: lib/**/*.s,*.o so `ww test`
  droppings stay untracked.

`make test` (26/26), `make bootstrap` (ww2≡ww3≡ww4), and per-module
`ww test` all pass. cgen output is byte-identical across cstage and
selfhost for every repro that previously diverged.

lib/hash/crc32/crc32.ww

@@ -0,0 +1,36 @@
+// hash/crc32 — CRC-32 checksum. Pure ww.
+//
+// Same shape as lib/hash/crc16: per-byte inline polynomial shift,
+// no precomputed table. Slower than Hare's table-driven path by ~8x
+// per byte but produces identical answers.
+
+def IEEE: u32       = 0xEDB88320u32;	// gzip, PNG, zip, Ethernet
+def CASTAGNOLI: u32 = 0x82F63B78u32;	// iSCSI, SCTP, SSE4.2
+def KOOPMAN: u32    = 0xEB31D82Eu32;	// small datasets
+
+// sum32 — fold `buf` under `poly` (reversed form). Initial cval is
+// ~0u32; per byte we mix in the low byte via 8 polynomial shifts and
+// XOR with the high three bytes shifted down.
+export fn sum32(buf: []u8, poly: u32) u32 = {
+	let c: u32 = 0xFFFFFFFFu32;
+	let i: i32 = 0;
+	for (i < buf.len) {
+		let t: u32 = (c & 0xFFu32) ^ (buf[i]: u32);
+		let z: i32 = 0;
+		for (z < 8) {
+			if ((t & 1u32) == 1u32) {
+				t = (t >> 1u32) ^ poly;
+			} else {
+				t = t >> 1u32;
+			};
+			z += 1;
+		};
+		c = t ^ (c >> 8u32);
+		i += 1;
+	};
+	return ~c;
+};
+
+export fn sum32ieee(buf: []u8) u32       = { return sum32(buf, IEEE); };
+export fn sum32castagnoli(buf: []u8) u32 = { return sum32(buf, CASTAGNOLI); };
+export fn sum32koopman(buf: []u8) u32    = { return sum32(buf, KOOPMAN); };

lib/hash/crc32/crc32_test.ww

@@ -0,0 +1,62 @@
+use crc32;
+
+fn putstr(s: str, into: []u8, off: i32) i32 = {
+	let i: i32 = 0;
+	for (i < s.len) {
+		into[off + i] = s[i];
+		i += 1;
+	};
+	return off + s.len;
+};
+
+fn check(s: str, ieee: u32, cast: u32, koop: u32) void = {
+	let arr: [128]u8;
+	let n: i32 = putstr(s, arr[0:128], 0);
+	let buf: []u8 = arr[0:n];
+	if (crc32.sum32ieee(buf)       != ieee) { let _: i32 = 1/0; };
+	if (crc32.sum32castagnoli(buf) != cast) { let _: i32 = 1/0; };
+	if (crc32.sum32koopman(buf)    != koop) { let _: i32 = 1/0; };
+};
+
+@test fn vec_empty() void = {
+	let arr: [1]u8;
+	let buf: []u8 = arr[0:0];
+	if (crc32.sum32ieee(buf)       != 0u32) { let _: i32 = 1/0; };
+	if (crc32.sum32castagnoli(buf) != 0u32) { let _: i32 = 1/0; };
+	if (crc32.sum32koopman(buf)    != 0u32) { let _: i32 = 1/0; };
+};
+
+@test fn vec_fire() void = {
+	check("Give a man a fire, they be warm for a day",
+		4026734998u32, 2112273292u32, 1263149916u32);
+};
+
+@test fn vec_setfire() void = {
+	check("Set a man on fire, they be warm for the rest of their life",
+		3931092334u32, 4172943610u32, 3071632577u32);
+};
+
+@test fn vec_blm() void = {
+	check("Black lives matter",
+		2370964079u32, 2068416418u32, 4151357773u32);
+};
+
+@test fn vec_unix() void = {
+	check("UNIX is simple and coherent",
+		3254252081u32, 1650777601u32, 340189632u32);
+};
+
+@test fn vec_gnu() void = {
+	check("GNU's not UNIX",
+		224734747u32, 200511816u32, 332335539u32);
+};
+
+export fn main() i32 = {
+	vec_empty();
+	vec_fire();
+	vec_setfire();
+	vec_blm();
+	vec_unix();
+	vec_gnu();
+	return 0;
+};