lib/path: #138 c2-stack buffer port — buffer/push/appendnorm/appendlit/string/isroot/set + dot/dotdot, cstage @test

Faithful realignment of lib/path to Hare's buffer-centric API
(ref/hare/path/{stack,buffer}.ha). Replaces the old str-only path.ww
wholesale (zero consumers). Scope = stack-core: init() deferred (returns
~4KB (buffer|error), rides the #40 arc / #147); abs/dirname/basename land
in c3; extension/join dropped.

appendlit uses the #145 slice-copy-assign arm (buf.buf[lo:hi]=bs), no hand
loop. dot/dotdot are faithful module-global []u8 (D2 #148). MAX =
os.PATH_MAX-1. Divergences (size->i32 indices, frombytes, module-global
consts) cited inline.

Tests: lib/path/pathtest.ww (table-driven), wired at test/wcc/989_path_run.c;
push rows mirror stack.ha:107-111 verbatim incl the restored "/d"
intermediate. Slot 989 (overflow bucket) since 970 is taken.

C-first: cstage @test green; wwstage byte-id deferred to the #125 batch.
path classified M_WWREJECT in 989_lib_byteid (w6c_ww rejects the module-
global slice consts = #120/#29; + the #148 twin #151) — self-graduates back
to M_ID the day wwstage accepts. path moved off the 900_stdlib standalone
list (import-dependent: os.PATH_MAX def-dim + match over imported error
types; coverage at 989_path_run), per the bytes/fmt/os precedent.

Gate: all 324 passed, byte-id 990-997 green, w6c/w6c_ww unchanged
(lib-only, non-embedded).

lib/path/pathtest.ww

@@ -0,0 +1,123 @@
+// pathtest — exercises lib/path (c2-stack subset). Run with
+// `out/bin/ww run lib/path/pathtest.ww`. Same signalled-then-fail()
+// -with-+10 pattern as bytes / getopt tests: a non-zero exit code
+// pinpoints the failing scenario.
+//
+// Vectors mirror Hare's @test fns in ref/hare/path/stack.ha:77-119
+// and buffer.ha. The absolute rows (Hare's `local("/")`-seeded) DEFER
+// to c3 (abs/local unimplemented); only the relative rows run here.
+//
+// Parallel `[N]str` row arrays (rather than `[N]struct{...}`) sidestep
+// the cstage cgen chained `arr[i].field` store gap (getopttest.ww:6).
+// push is stateful, so each row is applied in sequence to one buffer;
+// the table holds {segment, expected-string} pairs (stack.ha:36-42 is
+// the normalization spec these rows encode).
+
+package path;
+
+import path;
+import os;
+
+let signalled: i32 = 0;
+fn fail() void = { os.exit(signalled + 10); };
+
+// ---- push() + appendnorm normalization --------------------------------
+// ref/hare/path/stack.ha:77-119 (relative rows only; absolute local()
+// rows defer to c3). The dot/dotdot handling exercised here IS the
+// appendnorm normalization spec (stack.ha:36-42), so no separate
+// appendnorm @test is needed.
+
+@test fn push_cases() void = {
+	let buf = buffer { ... };
+	if (string(&buf) != ".") { fail(); };
+
+	// current-dir + parent-dir invariants (stack.ha:82-88). Single
+	// segment per row → table-driven sequential apply.
+	let segs: [5]str;
+	let wants: [5]str;
+	segs[0]="";   wants[0]=".";
+	segs[1]=".";  wants[1]=".";
+	segs[2]=".."; wants[2]="..";
+	segs[3]="";   wants[3]="..";
+	segs[4]=".";  wants[4]="..";
+	let i: i32 = 0;
+	for (i < 5) {
+		if (push(&buf, segs[i])! != wants[i]) { fail(); };
+		i += 1;
+	};
+
+	// set(&buf) resets to "." (stack.ha:91). set forwards an EMPTY
+	// variadic into push (KEN-oracle flag 1: zero-arg gather+forward).
+	if (set(&buf)! != ".") { fail(); };
+
+	// regular path + parent (stack.ha:101-104, minus the local() row).
+	let segs2: [3]str;
+	let wants2: [3]str;
+	segs2[0]="foo"; wants2[0]="foo";
+	segs2[1]=".";   wants2[1]="foo";
+	segs2[2]="..";  wants2[2]=".";
+	let k: i32 = 0;
+	for (k < 3) {
+		if (push(&buf, segs2[k])! != wants2[k]) { fail(); };
+		k += 1;
+	};
+
+	// multi-segment (stack.ha:107-111). Variadic arity varies (1 or 2)
+	// → inline.
+	if (push(&buf, "a", "b")!  != "a/b") { fail(); };
+	if (push(&buf, "..", "c")! != "a/c") { fail(); };
+	if (push(&buf, "..")!      != "a")   { fail(); };
+	// stack.ha:110; local()→literal, SEP='/' on Linux, proper local() rides c3
+	if (push(&buf, "/d")!      != "a/d") { fail(); };
+	if (push(&buf, "..", "..")! != ".")  { fail(); };   // stack.ha:111
+
+	// leading-SEP segment into an EMPTY buffer exercises push's
+	// `j==0 && buf.end==0` root-seed arm (stack.ha:18-20), unreached
+	// by the relative rows above. Hare covers it via local("/")-seeded
+	// rows that defer to c3; literal "/foo" stands in (SEP='/' on Linux).
+	if (set(&buf)! != ".") { fail(); };
+	if (push(&buf, "/foo")! != "/foo") { fail(); };
+};
+
+// ---- isroot() ---------------------------------------------------------
+// ref/hare/path/buffer.ha:44-51 (*buffer arm). Heterogeneous seeds
+// (manual root seed vs push) → inline rather than a uniform table.
+
+@test fn isroot_cases() void = {
+	let buf = buffer { ... };
+	// empty buffer (end 0)
+	if (isroot(&buf)) { fail(); };
+
+	// "/" — root. local() is c3, so seed the SEP byte directly.
+	buf.buf[0] = SEP; buf.end = 1;
+	if (!isroot(&buf)) { fail(); };
+	if (string(&buf) != "/") { fail(); };
+
+	// "foo" — relative, not root.
+	buf.end = 0;
+	if (push(&buf, "foo")! != "foo") { fail(); };
+	if (isroot(&buf)) { fail(); };
+
+	// "/foo" — absolute but not root.
+	buf.buf[0] = SEP; buf.end = 1;
+	if (push(&buf, "foo")! != "/foo") { fail(); };
+	if (isroot(&buf)) { fail(); };
+};
+
+// ---- string() ---------------------------------------------------------
+// ref/hare/path/buffer.ha:28-31. Empty buffer views as "."; a filled
+// buffer views the byte prefix.
+
+@test fn string_cases() void = {
+	let buf = buffer { ... };
+	if (string(&buf) != ".") { fail(); };
+	if (push(&buf, "foo")! != "foo") { fail(); };
+	if (string(&buf) != "foo") { fail(); };
+};
+
+export fn main() i32 = {
+	signalled = 1; push_cases();
+	signalled = 2; isroot_cases();
+	signalled = 3; string_cases();
+	return 0;
+};