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.

Makefile

@@ -113,7 +113,7 @@ $(BIN)/wwdump_ww: selfhost/cmd/wwdump/main.ww \
 		selfhost/cmd/wcc/cgen.ww selfhost/cmd/wcc/cgenexpr.ww \
 		selfhost/cmd/wcc/cgenstmt.ww selfhost/cmd/wcc/cgenutil.ww \
 		selfhost/cmd/wcc/cgendecl.ww \
-		lib/os/os.ww lib/strconv/strconv.ww \
+		lib/os/os.ww lib/strconv/strconv.ww lib/ascii/ascii.ww lib/fmt/fmt.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \
@@ -134,7 +134,7 @@ $(BIN)/w6c_ww: selfhost/cmd/w6c/main.ww \
 		selfhost/cmd/wcc/cgen.ww selfhost/cmd/wcc/cgenexpr.ww \
 		selfhost/cmd/wcc/cgenstmt.ww selfhost/cmd/wcc/cgenutil.ww \
 		selfhost/cmd/wcc/cgendecl.ww \
-		lib/os/os.ww lib/strconv/strconv.ww \
+		lib/os/os.ww lib/strconv/strconv.ww lib/ascii/ascii.ww lib/fmt/fmt.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \