cmd+selfhost+lib+test: directory-as-module enumeration in driver (#22)

Replace the cmd/ww + selfhost driver's file-walk import resolver
with true directory enumeration. `import encoding.utf8;` now finds
the lib/encoding/utf8/ directory and concatenates every *.ww file
in it (excluding *test.ww and the driver's *.combined.ww artifacts)
in byte-wise sorted order, instead of just finding the single
lib/encoding/utf8/utf8.ww file. Mirrors Hare's
hare/module/srcs.ha:183 _findsrcs minus tag handling.

Lookup order in both stages: (1) <dir>/<dot-as-slash>/ as directory
→ enumerate. (2) <dir>/<dot-as-slash>.ww as file. The legacy
<dir>/<name>/<name>.ww shape from #18's retained divergence is
dropped per rule-9 Hare-fidelity — Hare has no foo/foo.ha fallback;
a module IS the directory.

Symmetric across cstage (cmd/ww/main.c via opendir+qsort+stat) and
wwstage (selfhost/cmd/ww/main.ww via existing lib/os.getdents64 +
os.stat — no new lib/os surface needed; the rundirtests() walker
in main.ww from #18 was the model). Bootstrap ww2.s==ww3.s==ww4.s
byte-identical post-change.

Bundling justification (rule 11): strict-same-package validation is
bundled because the failure mode is dir-enum's own (a non-dir-enum
compilation unit cannot trigger mismatch across enumerated files).
The natural enforcement site is the driver — the parser can't
distinguish dir-enum concat from file-walk concat. Both stages
peek each file's first `package <name>;` line in expand_dir /
expanddir and exit(1) on mismatch with a precise error pointing
at the offending file. Hare's hare/module/srcs.ha:131 has the
same constraint via its README gate. Other half of #23 (strict
missing-package error tightening — 63 inline-source test wrappers
blocker) stays deferred per its filing.

Parser side (cmd/wcc/parse.c parseuse + lib/ww/parse/decl.ww
parseuse): n->str now carries only the LEAF identifier from a
dotted import. With the driver translating the full dotted path
to a directory walk, the checker only needs the package bareword
(last component) for the N_USE → decl disambiguation walk in
check.c's src_imports / decl_mod. Mirrors Hare's
`use encoding::utf8;` → `utf8::name` semantics
(ref/hare/hare/ast/import.ha:7).

Migration: lib/ww/sym.ww drops `import typ; import ast;`;
lib/ww/parse/parse.ww drops `import expr; import stmt; import
decl;`; lib/ww/lex/lex.ww drops `import tok;` — all sibling
imports auto-resolve via the new dir-enum when callers import the
package directory. lib/strings/, lib/encoding/utf8/utf8test.ww
migrate `import utf8;` → `import encoding.utf8;`. Makefile drops
-I lib/encoding/utf8 stopgap from wwdump_ww + w6c_ww. Seven test
wrappers (700_e2e, 966_strings_run, 970_fmt_run, 971_log_run,
972_fnmatch_run, 982_getopt_run, 990_selfhost) and 995_self_rebuild
drop the -I lib/encoding/utf8 runtime stopgap.

Tests: new 737_direnum C wrapper + test/wcc/data/direnum/ fixtures
pin (a) cross-pkg multi-file dir-enum build at runtime (both stages
must succeed) and (b) strict-same-package mismatch error (both
stages must surface "differs from" + exit non-zero). 738_module_decl
gains row 6 pinning the n_use->str leaf-only storage post-parser
change.

Retained workaround at selfhost/cmd/ww/main.ww expanddir loop:
`names[i][k]` nested-deref-then-index split into
`let nm: *u8 = names[i]; nm[k]` because wwstage cgen miscompiles
the chained form (treats inner u8 element as 8B sizeof *u8 instead
of 1B sizeof u8: extra MOVQ $8 + IMULQ on the inner index, MOVQ
instead of MOVZBQ load). Inline rule-8 WHY comment cites task #24
(wwstage cgen chained-index inner element size on **T). Two-step
form routes through the bare-pointer index path which both stages
handle byte-identically.

Class A wwstage cgen UNDER (chained-index inner element size on
**T) surfaced first time the codebase exercises the **T[i][k]
shape via enumeratedir() — corpus-coverage-blind landmine pattern,
same family as the trio (#27/#28/#31) from STATUS-5.

112/112 ok. ww2 == ww3 == ww4 byte-id holds.

lib/ww/parse/decl.ww

@@ -6,6 +6,11 @@ import os;
 import mem;
 import tok;
 
+// `import encoding.utf8;` — the driver resolves the dotted path to
+// a directory; only the leaf (`utf8`) is needed downstream as the
+// module bareword for n_use → decl disambiguation, mirroring Hare's
+// `use encoding::utf8;` → `utf8::name` (ref/hare/hare/ast/import.ha:7
+// stores `[]str` but identifier-resolution uses the last component).
 fn parseuse(p: *parser) *node = {
 	let pf: str = p.curfile;
 	let pl: i32 = p.curline;
@@ -13,32 +18,13 @@ fn parseuse(p: *parser) *node = {
 	advance(p);	// past `use`
 	let n: *node = newnode(p.a, nkind.N_USE, pf, pl, pc);
 	n.nmod = p.curmod;
-	// Accept a dotted import path: `use encoding.utf8;` — capture the
-	// full dotted form on n.str. Leaf-only SK_USE install lives in
-	// the check stage; the lexer-side join happens here.
-	let id: str;
-	expectident(p, &id);
-	n.str = id;
+	let leaf: str;
+	expectident(p, &leaf);
 	for (p.curkind == tkind.TK_DOT) {
 		advance(p);	// past `.`
-		let seg: str;
-		expectident(p, &seg);
-		// Concatenate id + "." + seg into a fresh str. Plan-9
-		// separator per user pick over Hare's `::`.
-		let total: i32 = n.str.len + 1 + seg.len;
-		let buf: *u8 = amalloc(p.a, total: u64 + 1u64): *u8;
-		let i: i32 = 0;
-		for (i < n.str.len) { buf[i] = n.str[i]; i += 1; };
-		buf[i] = 46u8;	// '.'
-		i += 1;
-		let j: i32 = 0;
-		for (j < seg.len) { buf[i + j] = seg[j]; j += 1; };
-		buf[total] = 0u8;
-		let joined: str;
-		joined.ptr = buf;
-		joined.len = total;
-		n.str = joined;
+		expectident(p, &leaf);
 	};
+	n.str = leaf;
 	expecttok(p, tkind.TK_SEMI, "expected ';' after use");
 	return n;
 };

lib/ww/parse/parse.ww

@@ -12,12 +12,12 @@
 
 package parse;
 
+// Sibling imports (expr, stmt, decl) auto-resolve via task #22
+// dir-enum when callers `import parse;` (which dir-enums
+// lib/ww/parse/).
 import os;
 import mem;
 import tok;
-import expr;
-import stmt;
-import decl;
 
 type parser = struct {
 	l:    *lex,