rm dead combined.ww + retarget 901 to sep-feed; sweep amalgamator remnants (M4 E4, #90)

The E3 flip (#87) made sep the sole compile path and deleted the
combined.ww writer, leaving the six committed *.combined.ww files dead.
Remove them and the last references to the retired amalgamator.

- rm the 6 tracked *.combined.ww (selfhost/cmd/{w6a,w6c,w6l,ww,wwdump}/
  main.combined.ww + selfhost/test/smoke.combined.ww). Verified no live
  build path or gate still feeds one as compiler INPUT.
- 901_asserttyped_gap: its 5 combined.ww gap fixtures were the last
  combined.ww INPUT consumers (4 already missing/vacuous post-flip, only
  smoke.combined.ww still fed). Retarget all 5 to sep-feed via a
  resolveunit helper (whole-package-dir copy -> `ww build --sep` ->
  <stem>.sepwork/__root.unit.ww), mirroring 990's #89 pattern; the 3
  import-free test fixtures stay raw-fed. All 8 counts hold at 0 (A-D
  coverage, vacuous since the flip, is live again).
- INV-2 (the driver's unresolvable-import-is-fatal guard) is KEPT; only
  its "Mirrors the deleted expand" lineage tail is swept. The #110
  combined_ww_fresh freshness gate was already removed in #89 (5f85852).
- Sweep dangling amalgamator lineage comments (build_one/buildone/expand/
  peek_package/peekpackage + stale combined.ww/combined intermediates)
  in cmd/ww/main.c + selfhost/cmd/ww/main.ww, symmetrically (rule-10),
  and the stale Makefile combined.ww test-comments (enumcap bigmod.unit.ww
  + 784/787/792/794/848 sep .s cmp + make-smoke sep self-compile).

Closes M4 and epic #22. all 445 pass; 990/993/994/995 byte-id HOLD;
sizelint clean.

selfhost/cmd/ww/main.ww

@@ -195,7 +195,7 @@ type strnode = struct {
 };
 
 type expctx = struct {
-	out:   i32,	// fd we're writing the combined source to
+	out:   i32,	// fd we're writing the sep unit source to
 	dirs:  *u8,	// ":"-separated search path (NUL-terminated)
 	visit: *strnode,
 };
@@ -326,9 +326,9 @@ fn locateimport(dirs: *u8, name: *u8, namelen: u64,
 };
 
 // Filter for dir enumeration: keep `*.ww` minus `*test.ww` and the
-// `*.combined.ww` driver-generated concat artifacts (the previous
-// build leaves them in the source tree; they parse-error when
-// re-included). Returns true to keep.
+// `*.combined.ww` legacy amalgamator artifacts (pre-#87): they
+// parse-error when re-included, and the sep driver no longer writes
+// them. Returns true to keep.
 fn dirfilekeep(name: *u8, nlen: u64) bool = {
 	// nlen<=3 guard kept: a bare ".ww" (len 3) is rejected here but
 	// would pass strings.hassuffix(".ww"); preserves cstage parity.
@@ -372,7 +372,7 @@ fn enumeratedir(dirpath: *u8) (**u8, i32) = {
 	if (fd < 0) { return nil: **u8, 0; };
 	// #65: grow-dynamic (mirror cstage enumerate_dir_ww realloc-doubling,
 	// cmd/ww/main.c:209). The old fixed 256-name cap silently dropped every
-	// eligible file past it, diverging the combined.ww from cstage on a
+	// eligible file past it, diverging the package unit from cstage on a
 	// module dir with >256 sources.
 	let cap: i32 = 8;
 	let names: []*u8 = alloc([], cap: u64)!;
@@ -542,11 +542,10 @@ fn appendlit(stem: *u8, suffix: str) *u8 = {
 	return buf.ptr;
 };
 
-// linker flags (-L<dir>, -l<name>) grouped as one struct so buildone's
+// linker flags (-L<dir>, -l<name>) grouped as one struct so buildonesep's
 // param list stays readable. (The earlier note here claimed a 6-argument
 // wwstage calling-convention cap; that is stale — emittuplerowrelocs in
-// cgen.ww takes 7 params and self-compiles green, and buildone itself now
-// takes 7.)
+// cgen.ww takes 7 params and self-compiles green.)
 type lflags = struct {
 	libdirs:   **u8,
 	nlibdirs: i32,
@@ -559,7 +558,7 @@ type lflags = struct {
 // Port of cmd/ww/main.c build_one_sep (task #46/c3). The `--sep` path
 // materializes each imported package's `.wwi` interface and compiles
 // every package on its own (`w6c -c`), then flat-links the `.o` set.
-// combined.ww stays the DEFAULT live path; --sep is additive.
+// Separate compilation is the SOLE build path (E3-C1 flip, task #87).
 //
 // Each w6c pass is BOTH consumer (reads dep `.wwi` as import scope) AND
 // producer (writes this package's `.wwi` via -I). Reverse-topo order
@@ -638,7 +637,7 @@ fn sepfname(g: *sepgraph, pi: i32, scratch: *u8, suffix: str) *u8 = {
 // genuinely-missing import from one satisfied by an INLINE package in the
 // same single-file multi-package unit (`package aa; ... package main;
 // import aa;`). Scans EVERY line (comment-skip) — not just the first
-// package decl (peekpackage stops there). Decision byte-identical to
+// package decl. Decision byte-identical to
 // cstage so the skip/fatal choice + driver output match (rule 10).
 fn unithaspackage(buf: *u8, buflen: u64, leafp: *u8, leafn: u64) bool = {
 	let p: u64 = 0u64;
@@ -756,10 +755,10 @@ fn sepscanfile(g: *sepgraph, pi: i32, file: *u8, searchpath: *u8,
 				// when the package is defined INLINE in the same unit
 				// (single-file multi-package). leaf = last dotted
 				// component; inline `package <leaf>` -> skip (the
-				// checker binds it), else fatal. E3-C1: the combined
-				// path that owned the genuine-missing case is gone, so
-				// the sep producer enforces it here (INV-2). cstage
-				// twin; fatal text identical.
+				// checker binds it), else fatal. E3-C1 (#87): the
+				// legacy amalgamator that owned the genuine-missing
+				// case is gone, so the sep producer enforces it here
+				// (INV-2). cstage twin; fatal text identical.
 				let lstart: u64 = 0u64;
 				let lk: u64 = 0u64;
 				for (lk < idn) {
@@ -1377,7 +1376,7 @@ fn buildonesep(selfdir: *u8, src: *u8, entryisdir: i32, out: *u8,
 		cstrseal(dotdotlib.ptr, off);
 	};
 
-	// Source directory (mirrors buildone).
+	// Source directory.
 	let srcd: []u8 = alloc([], (os.PATH_MAX: u64))!;
 	srcd.len = os.PATH_MAX;
 	if (entryisdir != 0) {
@@ -1422,7 +1421,7 @@ fn buildonesep(selfdir: *u8, src: *u8, entryisdir: i32, out: *u8,
 		cstrseal(searchpath.ptr, off);
 	};
 
-	// Stem for the scratch dir (mirrors buildone).
+	// Stem for the scratch dir.
 	let stem: []u8 = alloc([], (os.PATH_MAX: u64))!;
 	stem.len = os.PATH_MAX;
 	if (entryisdir != 0) {
@@ -1462,8 +1461,8 @@ fn buildonesep(selfdir: *u8, src: *u8, entryisdir: i32, out: *u8,
 	// #79 (-T): lib/test is the synth main's `test.run` callee but @test
 	// files never `import test;`. Inject it as a direct dep of the root so
 	// sepscanpkg pulls test + its transitive deps; the producer adds -T to
-	// the root and `test.run` links against test's `.a`. Mirrors the
-	// combined path's auto-bundle (buildone istest) and sepscanfile dedup.
+	// the root and `test.run` links against test's `.a` — via the
+	// sepscanfile dedup-guarded dep append.
 	if (istest != 0) {
 		let td: i32 = 0;
 		let tp: *u8 = locateimport(searchpath.ptr, "test".ptr, "test".len: u64, &td);
@@ -1575,7 +1574,7 @@ fn buildonesep(selfdir: *u8, src: *u8, entryisdir: i32, out: *u8,
 		// Wrap each DEP package's `.o` in its own deterministic `.a`
 		// (5a). The ROOT stays a positional `.o` (force-loaded — it's
 		// the build target), so `main` is defined before any archive is
-		// processed (mirrors buildone's root treatment). The link
+		// processed. The link
 		// consumes `.o`/`.a`, never `.wwi`.
 		if (pi != root) {
 			let apath: *u8 = sepfname(g, pi, scratch, ".a");
@@ -2069,7 +2068,7 @@ fn dorun(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
 	lf.nlibdirs = nlibdirs;
 	lf.libs = libs.ptr;
 	lf.nlibs = nlibs;
-	// objstem = tmp → intermediates at /tmp/ww_run_<pid>.{s,o,combined.ww},
+	// objstem = tmp → intermediates under /tmp/ww_run_<pid>.sepwork/,
 	// never next to the source (T3).
 	if (buildonesep(selfdir, resolved, isdir, tmp.ptr, tmp.ptr, incs.ptr, &lf, 0i32) != 0) {
 		os.remove(pathstr(tmp.ptr));
@@ -2104,8 +2103,8 @@ fn dorun(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
 fn runsingletest(selfdir: *u8, src: *u8, incs: *u8, compileonly: i32, outstem: *u8, pattern: *u8) i32 = {
 	let tmp: []u8 = alloc([], (os.PATH_MAX: u64))!;
 	tmp.len = os.PATH_MAX;
-	// -o redirects the binary + its combined (objstem, T3) to <stem>; the
-	// default temp keeps the combined next to the source as before.
+	// -o redirects the binary + its sepwork intermediates (objstem, T3) to
+	// <stem>; the default temp keeps them next to the source as before.
 	let outp: *u8 = nil;
 	let objstem: *u8 = nil;
 	if (outstem != nil) {
@@ -2248,7 +2247,7 @@ fn dotest(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
 	cstrseal(incs.ptr, 0u64);
 
 	// -c (compile-only) + -o <stem>: build the test binary + its lib/test-
-	// inclusive combined WITHOUT running it, for the byte-id gates. cstage
+	// inclusive sep unit WITHOUT running it, for the byte-id gates. cstage
 	// twin: cmd/ww/main.c do_test (error wording identical).
 	let compileonly: i32 = 0;
 	let outstem: *u8 = nil;