wcc,ww: flip driver to separate-compilation only; delete build_one amalgamator (M4 E3-C1, #22)

build_one_sep (per-package compile + .wwi interfaces + link) becomes the
sole build path. do_build/do_run/do_test and the ww twins all route
through it; --sep is now an accepted no-op and the run-rejects-sep guard
is removed.

Deleted the single-file amalgamator, both stages: build_one, expand,
expand_dir, peek_package (+ the wwstage twins + strictpkgmismatch).
unit_has_package is retained -- the sep scan loop's inline-package check
needs it. The sep-shared helpers (enumerate_dir_ww, locate_import*,
import_path_form, ImportSet, and ww counterparts) stay; they back the
surviving sep path.

Restores missing-package enforcement under sep by construction: the sep
scan loop loudly rejects an unresolvable import (cannot find package
<name>) unless the package is defined inline in the same unit -- matching
the deleted amalgamator and closing the silent-accept the flip would
otherwise introduce.

All 5 wwstage tools relink (each is built via the now-sep `ww build`);
emitted asm is byte-identical to the combined build per bootstrap input,
so the binary md5 delta is pure link layout, not codegen.

selfhost/cmd/ww/main.combined.ww is now stale and unregenerable (its
writer build_one is deleted); #90 deletes it next.

Test retargets folded in (rule-11 carve-out, #61/#133 precedent): each
asserts post-flip-only behavior, is un-pre-migratable unlike #93/#94/#103,
and splitting reddens one side. Closes #97.
- 989_slttypepref -> dir-package layout (xb imports xa so both same-leaf
  `invalid` types are in scope at xb.f); inline-multipackage was the
  amalgamator shape, deleted with the flip.
- 989_sepbuild_run -> run --sep now genuinely runs (exit 7), not the old
  loud-reject (exit 2); + a private per-pid WW_PKGCACHE so the cs/ww
  per-package byte-id compare on the shared real lib pkgs (rt/time/os) no
  longer races concurrent siblings on the global out/.pkgcache (the flip
  made sep the sole path, so every test now contends that cache).
- 737_direnum -> the deleted strictpkgmismatch "differs from" wording ->
  sep's "does not match import path" (shared substring, wwstage terser #68).
- 989_lib_byteid -> corpus-completeness scan excludes generated .sepwork
  scratch (the old `! -name '*.combined.ww'` exclude didn't cover the new
  sep artifact).

test/wcc/989_sepbuild_run.c

@@ -236,6 +236,18 @@ main(void)
 	runwait(cmd);
 	mkdir(td, 0755);
 
+	/* E3-C1: with the flip making sep the sole build path, every concurrent
+	 * test now writes the global out/.pkgcache; this test's cs/ww per-package
+	 * byte-id compare on the heavily-shared real lib pkgs (rt/time/os) then
+	 * races a sibling building the same pkg. Pin a private per-pid cache so
+	 * the cs and ww legs are isolated (system() builds inherit this env).
+	 * pkgcache writes are non-atomic (cp -f), the real torn-read bug; this
+	 * hermetic isolation is correct standalone, pending #104 (atomic
+	 * temp+rename, both stages). */
+	char cachedir[80];
+	snprintf(cachedir, sizeof cachedir, "%s/pkgcache", td);
+	setenv("WW_PKGCACHE", cachedir, 1);
+
 	char rootww[1024];
 	snprintf(rootww, sizeof rootww, "%s/root.ww", td);
 	if (write_file(rootww, root_src)) { fail++; goto out; }
@@ -339,13 +351,11 @@ main(void)
 		}
 	}
 
-	/* run --sep symmetry (#46 c3): `run` has no sep-compile-then-run
-	 * path (out of commit-3 scope), so BOTH stages must LOUD-REJECT
-	 * `run --sep` identically (rule 10) — not one silently ignore it.
-	 * The reject exits 2 (the driver's usage/flag-error code); assert
-	 * cs and ww agree AND actually rejected (exit 2, not a build+run
-	 * that happened to exit nonzero). This row is the gate-visible
-	 * guard whose absence let the divergence hide. */
+	/* run --sep symmetry: E3-C1 makes sep the sole compile path, so
+	 * `run` now genuinely sep-compiles-then-runs (the old "no sep run
+	 * path → loud-reject exit 2" is gone with the amalgamator). BOTH
+	 * stages must build+run the cross-boundary fixture identically
+	 * (rule 10) and exit EXPECT_EXIT — `--sep` is an accepted no-op. */
 	{
 		int rc_cs, rc_ww;
 		snprintf(cmd, sizeof cmd,
@@ -354,9 +364,10 @@ main(void)
 		snprintf(cmd, sizeof cmd,
 		    "%s/ww_ww run --sep %s >/dev/null 2>&1", bin, rootww);
 		rc_ww = runwait(cmd);
-		if (rc_cs != rc_ww || rc_cs != 2) {
+		if (rc_cs != rc_ww || rc_cs != EXPECT_EXIT) {
 			fprintf(stderr, "sepbuild FAIL: run --sep cs=%d ww=%d "
-			    "(both must loud-reject, exit 2)\n", rc_cs, rc_ww);
+			    "(both must run via sep, exit %d)\n", rc_cs, rc_ww,
+			    EXPECT_EXIT);
 			fail++;
 		}
 	}