test: retarget Pattern-B gates to sep .sepwork layout; 915 off private global (M4 E3, #93)

The driver flip moves build artifacts from next-to-source <stem>.s to a
.sepwork/ scratch dir. 29 Pattern-B gates now build `ww build --sep -o <stem>`
and read <stem>.sepwork/__root.s (multi-package gates concat all
<stem>.sepwork/*.s, since cross-package labels live in per-package .s).
All intermediates redirect to /tmp (WW_PKGCACHE + -o), so the corpus runs
parallel-safe with no source-tree pollution. Tests pass now (--sep is live)
and survive the flip.

915 additionally retargeted off strconv's PRIVATE left_shift_table (a let,
not export) — which separate compilation correctly hides — onto a test-local
package that exports its own probe table (#96). The combined path only linked
it via a single-unit private leak; encapsulation is now honored under sep.

Test-only; all 5 *_ww binaries HOLD. 989_m1mangle_run deferred (blocked by
#99, imported-package fn main mangling under sep).

test/wcc/989_m1mangle_sym.c

@@ -51,13 +51,23 @@ static const struct check checks[] = {
 	{ "TEXT utf8.runesz,",    0 },
 };
 
-/* build_s — build `src` to `<stem>.s` via `driver`; returns 0 on success. */
+/* build_s — build `src` via `driver`, leaving the unit asm at `<stem>.s`;
+ * returns 0 on success.
+ * #93 sep layout: `--sep -o <stem>` splits the asm across
+ * <stem>.sepwork/<pkg>.s (root in __root.s, the imported encoding.utf8 in
+ * encoding.utf8.s). The mangled `encoding.utf8.runesz` definition lives in
+ * the per-pkg .s and `TEXT main,` in __root.s, so we CONCAT all the
+ * per-unit .s (sorted glob order is deterministic) into a single <stem>.s
+ * for the needle scans + the rule-10 cmp downstream. WW_PKGCACHE is pinned
+ * beside <stem> to leave the shared out/.pkgcache untouched. */
 static int
 build_s(const char *driver, const char *src, const char *stem)
 {
 	char cmd[1024];
-	snprintf(cmd, sizeof cmd, "%s build -o '%s' '%s' 2>/dev/null",
-	    driver, stem, src);
+	snprintf(cmd, sizeof cmd,
+	    "WW_PKGCACHE='%s.pkgc' timeout 180 %s build --sep -o '%s' '%s' "
+	    "2>/dev/null && cat '%s.sepwork/'*.s > '%s.s'",
+	    stem, driver, stem, src, stem, stem);
 	return runwait(cmd);
 }
 
@@ -137,7 +147,8 @@ main(void)
 	}
 
 	char cmd[256];
-	snprintf(cmd, sizeof cmd, "rm -f '%s' '%s'* '%s'*", src, cstem, wstem);
+	/* #93: -rf to also drop the per-stem .sepwork scratch + pinned pkgc. */
+	snprintf(cmd, sizeof cmd, "rm -rf '%s' '%s'* '%s'*", src, cstem, wstem);
 	runwait(cmd);
 
 	if (fail) {