os: graduate filesize/readall/writeall to (i64 | oserror)

`type oserror = i64` carries -errno (Hare's errors::errno-shaped
named-i64). The three convenience wrappers move off the i64 = -1
sentinel and onto the tagged-union surface.

Callers updated across the selfhost (wwdump, w6c, w6a, w6l, ww
driver). The slurp paths in w6c/w6a/w6l/wwdump now match on the
filesize and readall results; the ELF-emitting writeall sites in
w6a/obj.ww are wrapped through two small local helpers (`wrn` for
"wrote N bytes ok?", `wrdrop` for fire-and-forget) so the existing
11-callsite write loop stays readable.

selfhost/test/smoke.ww kept using raw os.read instead of
os.readall: the 990 cgen-match probe compiles smoke.ww standalone
(no `use` expansion), and cross-module type references like
`os.oserror` can't be resolved in that mode.

Two selfhost-side gaps surfaced and got plugged:
- lib/ww/parse/parse.ww parsetype now collapses dotted type names
  (`pkg.Type` → single N_TNAME with the joined string), mirroring C
  parsetype's dotted-path loop. Local `joindotted` helper because
  there's no arena-based string-concat in the selfhost lib yet.
- selfhost/cmd/wcc/check.ww name-resolver applies the dotted-prefix
  rule from cmd/wcc/check.c's resolve_typename: split at the last
  dot, look up the head as a `use` import, then the leaf as a type.

selfhost/cmd/w6a/obj.ww

@@ -15,6 +15,26 @@ use os;
 use mem;
 use types;
 
+// Local wrappers around os.writeall's tagged return — collapse the
+// (i64 | oserror) back to a boolean / int sentinel for the
+// length-checked / fire-and-forget write patterns below.
+fn wrn(fd: i32, p: *u8, n: u64, want: i64) bool = {
+	let r: (i64 | os.oserror) = os.writeall(fd, p, n);
+	match (r) {
+	case let v: i64 => return v == want;
+	case let e: os.oserror => return false;
+	};
+	return false;
+};
+
+fn wrdrop(fd: i32, p: *u8, n: u64) void = {
+	let r: (i64 | os.oserror) = os.writeall(fd, p, n);
+	match (r) {
+	case let v: i64 => { };
+	case let e: os.oserror => { };
+	};
+};
+
 // ---- ELF constants ----------------------------------------------------
 def ELFCLASS64:   u8  = 2u8;
 def ELFDATA2LSB:  u8  = 1u8;
@@ -205,27 +225,27 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	wru16(eh.ptr, 60u64, NSECT);		// e_shnum
 	wru16(eh.ptr, 62u64, 5u16);		// e_shstrndx
 
-	if (os.writeall(fd, eh.ptr, 64u64) != 64i64) { return -1; };
+	if (!wrn(fd, eh.ptr, 64u64, 64i64)) { return -1; };
 	if (a.textlen > 0u64) {
-		if (os.writeall(fd, a.text, a.textlen) != a.textlen: i64) { return -1; };
+		if (!wrn(fd, a.text, a.textlen, a.textlen: i64)) { return -1; };
 	};
 	if (rela.n > 0u64) {
-		if (os.writeall(fd, rela.p, rela.n) != rela.n: i64) { return -1; };
+		if (!wrn(fd, rela.p, rela.n, rela.n: i64)) { return -1; };
 	};
 	if (sym.n > 0u64) {
-		if (os.writeall(fd, sym.p, sym.n) != sym.n: i64) { return -1; };
+		if (!wrn(fd, sym.p, sym.n, sym.n: i64)) { return -1; };
 	};
 	if (str_.n > 0u64) {
-		if (os.writeall(fd, str_.p, str_.n) != str_.n: i64) { return -1; };
+		if (!wrn(fd, str_.p, str_.n, str_.n: i64)) { return -1; };
 	};
 	if (shstr.n > 0u64) {
-		if (os.writeall(fd, shstr.p, shstr.n) != shstr.n: i64) { return -1; };
+		if (!wrn(fd, shstr.p, shstr.n, shstr.n: i64)) { return -1; };
 	};
 
 	// Pad to 8 before shdrs.
 	let written: u64 = EHDR_SZ + a.textlen + rela.n + sym.n + str_.n + shstr.n;
 	for ((written & 7u64) != 0u64) {
-		os.writeall(fd, &zero, 1u64);
+		wrdrop(fd, &zero, 1u64);
 		written += 1u64;
 	};
 
@@ -234,7 +254,7 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	// SHT_NULL
 	let sn: i32 = 0;
 	for (sn < 64) { shbuf[sn] = 0u8; sn += 1; };
-	os.writeall(fd, shbuf.ptr, 64u64);
+	wrdrop(fd, shbuf.ptr, 64u64);
 	// .text
 	sn = 0;
 	for (sn < 64) { shbuf[sn] = 0u8; sn += 1; };
@@ -244,7 +264,7 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	wru64(shbuf.ptr, 24u64, offtext);
 	wru64(shbuf.ptr, 32u64, a.textlen);
 	wru64(shbuf.ptr, 48u64, 1u64);	// sh_addralign
-	os.writeall(fd, shbuf.ptr, 64u64);
+	wrdrop(fd, shbuf.ptr, 64u64);
 	// .rela.text
 	sn = 0;
 	for (sn < 64) { shbuf[sn] = 0u8; sn += 1; };
@@ -257,7 +277,7 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	wru32(shbuf.ptr, 44u64, 1u32);	// sh_info = .text idx
 	wru64(shbuf.ptr, 48u64, 8u64);
 	wru64(shbuf.ptr, 56u64, RELA_SZ);
-	os.writeall(fd, shbuf.ptr, 64u64);
+	wrdrop(fd, shbuf.ptr, 64u64);
 	// .symtab
 	sn = 0;
 	for (sn < 64) { shbuf[sn] = 0u8; sn += 1; };
@@ -269,7 +289,7 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	wru32(shbuf.ptr, 44u64, 1u32);	// sh_info = one local (STN_UNDEF)
 	wru64(shbuf.ptr, 48u64, 8u64);
 	wru64(shbuf.ptr, 56u64, SYM_SZ);
-	os.writeall(fd, shbuf.ptr, 64u64);
+	wrdrop(fd, shbuf.ptr, 64u64);
 	// .strtab
 	sn = 0;
 	for (sn < 64) { shbuf[sn] = 0u8; sn += 1; };
@@ -278,7 +298,7 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	wru64(shbuf.ptr, 24u64, offstr);
 	wru64(shbuf.ptr, 32u64, str_.n);
 	wru64(shbuf.ptr, 48u64, 1u64);
-	os.writeall(fd, shbuf.ptr, 64u64);
+	wrdrop(fd, shbuf.ptr, 64u64);
 	// .shstrtab
 	sn = 0;
 	for (sn < 64) { shbuf[sn] = 0u8; sn += 1; };
@@ -287,7 +307,7 @@ export fn emitelf(a: *asm_, fd: i32) i32 = {
 	wru64(shbuf.ptr, 24u64, offshstr);
 	wru64(shbuf.ptr, 32u64, shstr.n);
 	wru64(shbuf.ptr, 48u64, 1u64);
-	os.writeall(fd, shbuf.ptr, 64u64);
+	wrdrop(fd, shbuf.ptr, 64u64);
 
 	return 0;
 };