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).

test/wcc/700_e2e.c

@@ -1748,8 +1748,14 @@ main(void)
 		char tmpdir[64];
 		snprintf(tmpdir, sizeof tmpdir, "/tmp/wwe2e_%d_d_%d", getpid(), i);
 		mkdir(tmpdir, 0755);
-		snprintf(cmd, sizeof cmd, "cd %s && %s/ww build %s",
-		    tmpdir, bin, src);
+		/* -I lib/encoding/utf8 for any fixture that pulls in fmt /
+		 * strconv / strings via `use` (transitive utf8.encoderune;
+		 * task #17). Unused -I is benign for fixtures that don't. */
+		char cwd700[1024];
+		if (getcwd(cwd700, sizeof cwd700) == NULL) { fail++; continue; }
+		snprintf(cmd, sizeof cmd,
+		    "cd %s && %s/ww build -I %s/lib/encoding/utf8 %s",
+		    tmpdir, bin, cwd700, src);
 		if (runwait(cmd) != 0) { fail++; continue; }
 
 		char outbin[128];