lib/strings+test: Hare port (dup/concat/trim/index/contains/has{pre,suf}fix/compare/utf8)

Hare-faithful index/predicate family per ref/hare/strings/{dup,
concat,trim,index,suffix,contains,compare,utf8}.ha. Non-variadic
subset (concat 2-arg, trim single-rune, contains single-needle)
pending task #16 — cstage variadic-pack drops .len on multi-field
element types; ship the Hare-faithful single-arg shape now, file
the variadic upgrade as follow-up. `sub` follow-up filed as #29
(commit 2 with iterator + utf8.chars relocation).

Surface: dup, concat, trim/trimprefix/trimsuffix (single rune),
hasprefix, hassuffix (both with (str|rune) sum needle),
byteindex, rbyteindex (both with (str|rune) sum needle),
contains (single str needle), compare, toutf8, fromutf8_unsafe,
runebytes helper. (str|rune) match arms route the rune via
utf8.encoderune into a [4]u8 scratch then bytes.index/rindex —
drew-devault's directive for clean Hare-fidelity over invented
ASCII-only rune-byte arms.

byteindex / rbyteindex rune-arm semantic correction —
corpus-coverage-blind unmask. Pre-existing impl scanned for
`r: u8` (broken for all rune values >0x7F since strings.ww first
landed; no caller exercised it). Replaced with utf8.encoderune-
based scan via runebytes helper. Severity-marker: silent
wrong-result for any non-ASCII rune needle, masked by zero
in-tree callers until lib/strings + utf8 chain pulled the shape
in.

Build-system propagation: lib/strings depends transitively on
lib/encoding/utf8 (via byteindex's rune arm). cmd/ww driver's
locate_import_in (cmd/ww/main.c:85) walks `<dir>/<name>.ww` and
`<dir>/<name>/<name>.ww` only — `use utf8;` doesn't find
lib/encoding/utf8/utf8.ww without explicit `-I lib/encoding/utf8`.
Propagated through 5 wwstage-tool Makefile targets + 7 test
wrappers + test/wcc/995_self_rebuild.c sprintf lines. Task #17
filed for the principled resolver fix (subdir walk vs Hare's
qualified `use encoding::utf8;` notation).

This commit chain (#15 strings) surfaced 7 cgen bugs during
landing: #16 cstage variadic-pack, #17 resolver nested-paths,
#27 aliaslookup leaf-collision, #22 zero-init !void/void-alias
let-decl, #15-cstage retscr SSoT name, #24 composite CALL return
as composite arg, #28 N_DOT calleeparams. All blocking ones
fixed (#16/#17 deferred-with-stopgap, others fixed in their
respective commits). Pre-flight + stop-and-surface discipline
held throughout — no workarounds shipped in stdlib.

Tests:
  - 966_strings_run drives lib/strings/stringstest.ww via ww run.
    15 @test fns: dup (alloc, multibyte), concat (empty, lopsided,
    multibyte), trim/ltrim/rtrim incl. 4-byte rune U+1D68A,
    hasprefix/hassuffix with (str|rune) incl. multibyte,
    byteindex/rbyteindex both arms 1/2/3/4-byte rune coverage,
    compare. Cited from ref/hare/strings/+test.ha where vectors
    apply.

100/100 ok. 995_self_rebuild stays green (ww2==ww3==ww4 byte-id).

Makefile

@@ -113,13 +113,14 @@ $(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/time/time.ww lib/strconv/strconv.ww lib/strings/strings.ww lib/ascii/ascii.ww lib/io/io.ww lib/memio/memio.ww lib/fmt/fmt.ww \
+		lib/os/os.ww lib/time/time.ww lib/strconv/strconv.ww lib/strings/strings.ww lib/bytes/bytes.ww lib/encoding/utf8/utf8.ww lib/ascii/ascii.ww lib/io/io.ww lib/memio/memio.ww lib/fmt/fmt.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \
 		-I $$PWD/../../lib/ww \
 		-I $$PWD/../../lib/ww/lex \
 		-I $$PWD/../../lib/ww/parse \
+		-I $$PWD/../../lib/encoding/utf8 \
 		-I $$PWD/../../selfhost/cmd/wcc \
 		$$PWD/../../selfhost/cmd/wwdump/main.ww
 	mv $(BIN)/main $@
@@ -134,13 +135,14 @@ $(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/time/time.ww lib/strconv/strconv.ww lib/strings/strings.ww lib/ascii/ascii.ww lib/io/io.ww lib/memio/memio.ww lib/fmt/fmt.ww \
+		lib/os/os.ww lib/time/time.ww lib/strconv/strconv.ww lib/strings/strings.ww lib/bytes/bytes.ww lib/encoding/utf8/utf8.ww lib/ascii/ascii.ww lib/io/io.ww lib/memio/memio.ww lib/fmt/fmt.ww \
 		$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(LIB)/libwwrt.a | $(BIN)
 	cd $(BIN) && ./ww build \
 		-I $$PWD/../../lib/ww \
 		-I $$PWD/../../lib/ww/lex \
 		-I $$PWD/../../lib/ww/parse \
+		-I $$PWD/../../lib/encoding/utf8 \
 		-I $$PWD/../../selfhost/cmd/wcc \
 		$$PWD/../../selfhost/cmd/w6c/main.ww
 	mv $(BIN)/main $@
@@ -268,6 +270,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_shlex_run $(BIN)/test_getenv_run $(BIN)/test_dirs_run \
 	  $(BIN)/test_stat_run $(BIN)/test_time_run \
 	  $(BIN)/test_intdiv_signed \
+	  $(BIN)/test_strings_run \
 	  $(BIN)/test_hex_run $(BIN)/test_utf8_run $(BIN)/test_bytes_run \
 	  $(BIN)/test_memio_run $(BIN)/test_temp_run $(BIN)/test_getopt_run \
 	  $(BIN)/test_base32_run $(BIN)/test_base64_run \
@@ -686,6 +689,10 @@ $(BIN)/test_bytes_run: test/wcc/967_bytes_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_strings_run: test/wcc/966_strings_run.c $(BIN)/ww $(BIN)/w6c \
+		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_memio_run: test/wcc/980_memio_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<