test: port the sep scratch-contract observer to ww; retire 989_sepscratch

sepscratch_test.ww pins the #59 build_one_sep scratch ownership on
both driver stages: build -o keeps <stem>.sepwork, `run` leaves no
/tmp/ww_run_<pid> root, and a foreign pre-existing .sepwork is
refused with exit exactly 1, sentinel intact. The C fork/execl pid
capture becomes `sh -c 'echo $$ ...; exec <drv> run ...'` -- exec
keeps the shell's pid and passes the full driver path as argv[0].

test/sep/sepscratch_test.ww

@@ -0,0 +1,127 @@
+package sepscratch_test;
+
+// #59 scratch-ownership contract of build_one_sep, both driver stages.
+// Port of the retired native carrier test/wcc/989_sepscratch_run.c;
+// every assertion preserved.
+//
+//  keep      — `build -o` scratch (<stem>.sepwork) SURVIVES the build
+//              (keepscratch: the byte-id gates read it).
+//  cleanrun  — `run` leaves no /tmp/ww_run_<pid> scratch root behind.
+//              The driver keys the root on its OWN pid, so the child is
+//              launched via `sh -c 'echo $$ > pid; exec <drv> ...'` —
+//              exec keeps the shell's pid, and the full driver path
+//              lands in argv[0] (the driver derives self_dir from it).
+//  collision — a pre-existing <stem>.sepwork belongs to someone else:
+//              build must refuse with exit EXACTLY 1 (not adopt), and
+//              the foreign sentinel must survive byte-for-byte.
+//
+// Dropped C machinery, not assertions: the fork/execl pid capture (the
+// sh exec trick above carries it) and the per-path unlink/rmdir
+// accounting (testenv.clean asserts the recursive removal).
+
+import os;
+import os.exec;
+import strings;
+import testenv;
+import time;
+
+fn fail(label: str, why: str) void = {
+	let m: str = strings.concat("sepscratch FAIL: ", label, " -- ", why,
+		"\n");
+	os.write(2, m.ptr, m.len: u64);
+	assert(false);
+};
+
+fn tmo() time.duration = {
+	return (240i64 * (time.second: i64)): time.duration;
+};
+
+// A no-import root: builds fast, still produces a real __root scratch.
+fn rootsrc() str = {
+	return strings.concat(
+		"package main;\n",
+		"fn main() i32 = { return 0; };\n");
+};
+
+@test fn keep() void = {
+	let td: str = testenv.fresh();
+	let rootww: str = strings.concat(td, "/root.ww");
+	testenv.writefile(rootww, rootsrc());
+	let drvs: []str = ["ww", "ww_ww"];
+	let i: i32 = 0;
+	for (i < drvs.len) {
+		let prog: str = strings.concat(td, "/prog_", drvs[i]);
+		let av: []str = [testenv.driver(drvs[i]), "build", "-o", prog,
+			rootww];
+		let co: testenv.commandout;
+		testenv.runcommand(td, td, strings.concat("build_", drvs[i]),
+			av, tmo(), &co);
+		if (co.termination != exec.termination.EXIT || co.code != 0) {
+			fail(drvs[i], "build -o failed");
+		};
+		if (!testenv.exists(strings.concat(prog, ".sepwork"))) {
+			fail(drvs[i], strings.concat("build -o scratch was removed ",
+				"(gates read it; keepscratch must hold)"));
+		};
+		i += 1;
+	};
+	testenv.clean(td);
+};
+
+@test fn cleanrun() void = {
+	let td: str = testenv.fresh();
+	let rootww: str = strings.concat(td, "/root.ww");
+	testenv.writefile(rootww, rootsrc());
+	let drvs: []str = ["ww", "ww_ww"];
+	let i: i32 = 0;
+	for (i < drvs.len) {
+		let pidf: str = strings.concat(td, "/pid_", drvs[i]);
+		let script: str = strings.concat("echo $$ > ", pidf, "; exec ",
+			testenv.driver(drvs[i]), " run ", rootww);
+		let av: []str = ["/bin/sh", "-c", script];
+		let co: testenv.commandout;
+		testenv.runcommand(td, td, strings.concat("run_", drvs[i]),
+			av, tmo(), &co);
+		if (co.termination != exec.termination.EXIT || co.code != 0) {
+			fail(drvs[i], strings.concat("run exit != 0 (must have ",
+				"built+run, proving non-vacuity)"));
+		};
+		let pid: str = strings.trim(testenv.readfile(pidf), '\n');
+		if (testenv.exists(strings.concat("/tmp/ww_run_", pid))) {
+			fail(drvs[i], "leaked the run scratch root");
+		};
+		i += 1;
+	};
+	testenv.clean(td);
+};
+
+@test fn collision() void = {
+	let td: str = testenv.fresh();
+	let rootww: str = strings.concat(td, "/root.ww");
+	testenv.writefile(rootww, rootsrc());
+	let drvs: []str = ["ww", "ww_ww"];
+	let i: i32 = 0;
+	for (i < drvs.len) {
+		let coll: str = strings.concat(td, "/collision_", drvs[i]);
+		let scr: str = strings.concat(coll, ".sepwork");
+		assert(os.mkdir(scr, 493) == 0);
+		let sentinel: str = strings.concat(scr, "/sentinel");
+		testenv.writefile(sentinel, "owned\n");
+		let av: []str = [testenv.driver(drvs[i]), "build", "-o", coll,
+			rootww];
+		let co: testenv.commandout;
+		testenv.runcommand(td, td, strings.concat("coll_", drvs[i]),
+			av, tmo(), &co);
+		if (co.termination == exec.termination.EXIT && co.code == 0) {
+			fail(drvs[i], "adopted a pre-existing .sepwork");
+		};
+		if (co.termination != exec.termination.EXIT || co.code != 1) {
+			fail(drvs[i], "collision refusal exit != 1");
+		};
+		if (!testenv.same(testenv.readfile(sentinel), "owned\n")) {
+			fail(drvs[i], "deleted or altered the foreign sentinel");
+		};
+		i += 1;
+	};
+	testenv.clean(td);
+};