lib: add missing Hare-stdlib functions (ascii/bytes/strings/path/endian)

ascii: valid, validstr, ispunct, isprint, iscntrl, isgraph, isblank,
strcasecmp.

bytes: hasprefix, hassuffix, rindex, rindexbyte, contains, reverse,
zero.

strings: rindex, sub, trimprefix, trimsuffix, ltrimbyte, rtrimbyte,
trimbyte. The byte-set trim is a single-byte subset of Hare's
`trim(input, exclude: rune...)`; no variadic ABI yet.

path: dirname, basename, extension, join. Owned-str returns where the
result isn't a borrowed view of the input (join).

endian: full Hare table — be/le get/put for u16/u32/u64 plus the
network-order htonu/ntohu pair extended to 32/64.

lib/CLAUDE.md rewritten to reflect the post-graduation policy
(tagged-union returns, owned-str returns, plan9 names, documented
deviations for non-graduating modules).

Makefile picks up lib/ascii and lib/fmt as wwdump_ww / w6c_ww deps so
lib-only edits regenerate the affected binaries.

selfhost/test/smoke.combined.ww

@@ -644,6 +644,61 @@ export fn isxdigit(c: rune) bool = {
 	return false;
 };
 
+// valid — `c` is in the 0..127 ASCII range.
+export fn valid(c: rune) bool = {
+	if (c < 0)   { return false; };
+	if (c > 127) { return false; };
+	return true;
+};
+
+// validstr — every byte in `s` is ASCII (0..127).
+export fn validstr(s: str) bool = {
+	let i: i32 = 0;
+	for (i < s.len) {
+		// High-bit test rather than `> 127u8`; both cgens lower
+		// the bitwise form identically. The `> u8` form picks
+		// JA vs JG depending on signed/unsigned dispatch.
+		if ((s[i] & 128u8) != 0u8) { return false; };
+		i += 1;
+	};
+	return true;
+};
+
+// iscntrl — control chars: 0..31 and 127.
+export fn iscntrl(c: rune) bool = {
+	if (c >= 0) { if (c <= 31) { return true; }; };
+	if (c == 127) { return true; };
+	return false;
+};
+
+// isblank — space and tab.
+export fn isblank(c: rune) bool = {
+	if (c == 32) { return true; };	// ' '
+	if (c == 9)  { return true; };	// '\t'
+	return false;
+};
+
+// isprint — printable: space through '~'.
+export fn isprint(c: rune) bool = {
+	if (c < 32)  { return false; };
+	if (c > 126) { return false; };
+	return true;
+};
+
+// isgraph — printable, non-space.
+export fn isgraph(c: rune) bool = {
+	if (c < 33)  { return false; };
+	if (c > 126) { return false; };
+	return true;
+};
+
+// ispunct — printable, non-alnum, non-space.
+export fn ispunct(c: rune) bool = {
+	if (!isgraph(c)) { return false; };
+	if (isalnum(c))  { return false; };
+	return true;
+};
+
 // tolower / toupper — fold ASCII case. Non-letters pass through.
 export fn tolower(c: rune) rune = {
 	if (isupper(c)) { return c + 32; };
@@ -655,6 +710,20 @@ export fn toupper(c: rune) rune = {
 	return c;
 };
 
+// strcasecmp — three-way ASCII case-insensitive compare.
+export fn strcasecmp(a: str, b: str) i32 = {
+	let n: i32 = a.len;
+	if (b.len < n) { n = b.len; };
+	let i: i32 = 0;
+	for (i < n) {
+		let ca: rune = tolower(a[i]: rune);
+		let cb: rune = tolower(b[i]: rune);
+		if (ca != cb) { return (ca - cb): i32; };
+		i += 1;
+	};
+	return a.len - b.len;
+};
+
 // MODULE: test
 // selfhost/test/smoke.ww — end-to-end smoke for the selfhost path.
 //