lib: split path_test into buffer/stack/posix tests per ref/hare/path

Mirror ref/hare/path's impl layout in the test siblings:
abs/isroot/string/local rows -> buffer_test.ww (ref/hare/path/
buffer.ha:28-77); push/parent/pop-split rows -> stack_test.ww
(ref/hare/path/stack.ha:9-180); dirname/basename rows ->
posix_test.ww (ref/hare/path/posix.ha:17-71).

Pure move: every @test block is byte-identical; only the section
banner lines are deleted (file names now carry them) and each
section's ref-cite WHY comment rides with its @test. The suite
header's Hare-vector/parallel-array WHY paragraphs ride with
stack_test.ww (push is their subject); the stale `ww run
lib/path/path_test.ww` sentence is dropped, not relocated.

Consumers: Makefile LIBRARY_TESTS entry replaced in place. The
byteid roster is untouched: lib/path is enrolled there as an
import-probe, not a fixture.

lib/path/posix_test.ww

@@ -0,0 +1,36 @@
+package path_test;
+
+import path;
+
+
+// ref/hare/path/posix.ha:51-71. Hare seeds each row through _local/local()
+// for cross-platform SEP; on ww (Linux, SEP='/') that is identity, so the
+// raw str literals stand in directly (local/_local defer to c3). POSIX
+// dirname/basename do NOT normalize the input (posix.ha:14-15,35), so no
+// seeding is needed. Parallel `[N]str` rows (cgen chained-field gap, as
+// above). Rows whose expected is a borrowed VIEW (not a "."/"/" literal)
+// compare a computed str to a literal — cstage-correct; wwstage mis-
+// compares via #146 (str== over a frombytes result), and this @test is
+// cstage-gated, so those rows are #146-deferred on wwstage only.
+
+@test fn dirname_basename_cases() void = {
+	let inputs:   [10]str;
+	let wantdir:  [10]str;
+	let wantbase: [10]str;
+	inputs[0]="usr";              wantdir[0]=".";          wantbase[0]="usr";
+	inputs[1]="usr/";             wantdir[1]=".";          wantbase[1]="usr";
+	inputs[2]="";                 wantdir[2]=".";          wantbase[2]=".";
+	inputs[3]="/";                wantdir[3]="/";          wantbase[3]="/";
+	inputs[4]="//";               wantdir[4]="/";          wantbase[4]="/";  // impl-defined
+	inputs[5]="///";              wantdir[5]="/";          wantbase[5]="/";
+	inputs[6]="/usr/";            wantdir[6]="/";          wantbase[6]="usr";
+	inputs[7]="/usr/lib";         wantdir[7]="/usr";       wantbase[7]="lib";
+	inputs[8]="//usr//lib//";     wantdir[8]="//usr";      wantbase[8]="lib";
+	inputs[9]="/home//dwc//test"; wantdir[9]="/home//dwc"; wantbase[9]="test";
+	let i: i32 = 0;
+	for (i < 10) {
+		assert(!(dirname(inputs[i]) != wantdir[i]));
+		assert(!(basename(inputs[i]) != wantbase[i]));
+		i += 1;
+	};
+};