lib/ascii: add strlower/strupper

Port ref/hare/ascii/string.ha strlower/strupper as the allocating entry
points: byte-wise ASCII case fold, equivalent to Hare's rune fold since
case-folding only touches bytes <0x80 and every UTF-8 multibyte byte is
>=0x80 (passes through unchanged, length-preserving). nomem arises only
from the allocation's `?`.

strlower_buf/strupper_buf are deferred: ww has no nomem-value form or
capacity-bounded static-append to express Hare's too-small-buffer path
(#230); restore the two-tier delegation when those land.

Divergence (rule 7): the empty-input fast path returns a nil/0 str
because ww's alloc([], 0) routes through nomem, whereas Hare allocs a
zero-length buffer and zero-loops; documented at the bypass site.

Test vectors mirror Hare's @test (ABC/abc/[[[/こ/empty/aB1z). Adds
lib/ascii/asciitest.ww + test/wcc/904_ascii_run.c (registered in the
Makefile TESTS list and a build rule). Regenerates the ascii-embedding
selfhost combined.ww amalgams (#110 freshness); the wwdump amalgam also
reorders the ascii block after strings to satisfy the new import edge.

lib/ascii/asciitest.ww

@@ -0,0 +1,41 @@
+// asciitest — exercises lib/ascii case folding. Run with
+// `ww run lib/ascii/asciitest.ww`. Same signalled-then-fail()-with-+10
+// pattern as bytestest: a non-zero exit pinpoints the failing scenario.
+//
+// Vectors mirror Hare's @test fn strcasecmp in ref/hare/ascii/string.ha.
+
+package ascii;
+
+import ascii;
+import os;
+
+let signalled: i32 = 0;
+fn fail() void = { os.exit(signalled + 10); };
+
+// checkfold — one table row: strlower(in) == lo and strupper(in) == up.
+fn checkfold(in: str, lo: str, up: str) void = {
+	match (ascii.strlower(in)) {
+	case let got: str => { if (got != lo) { fail(); }; };
+	case nomem        => { fail(); };
+	};
+	match (ascii.strupper(in)) {
+	case let got: str => { if (got != up) { fail(); }; };
+	case nomem        => { fail(); };
+	};
+};
+
+// ref/hare/ascii/string.ha:70 case-fold vectors. The こ row pins that a
+// UTF-8 multibyte sequence (all bytes >=0x80) passes through unchanged.
+@test fn strfold_cases() void = {
+	signalled = 100; checkfold("ABC", "abc", "ABC");
+	signalled = 101; checkfold("abc", "abc", "ABC");
+	signalled = 102; checkfold("[[[", "[[[", "[[[");
+	signalled = 103; checkfold("こ", "こ", "こ");
+	signalled = 104; checkfold("", "", "");
+	signalled = 105; checkfold("aB1z", "ab1z", "AB1Z");
+};
+
+export fn main() i32 = {
+	signalled = 1; strfold_cases();
+	return 0;
+};