test(e2e): 340 只数它自己那个目录里的 ninja (#557)

THE PREDICATE WAS RIGHT AND THE OBJECT WAS WRONG.

`count_ninja` counted every ninja on the machine. A baseline taken before the
build covers one that was ALREADY running; it does not cover one that appears
during the window, and a build server, a second checkout or a parallel test can
start one at any moment.

Measured in an ecosystem sandbox, which shares the host's PID namespace: an
unrelated session's ninja, in a different project entirely, made this assertion
fail on a build that has the fix.

Counting is now scoped by the process's working directory to the test's own
`$work` tree. Verified that scoping did not weaken it: the test still FAILS
against the released 2026.9.4.2, naming the orphan and its directory.

Co-authored-by: speak-agent <248744407+speak-agent@users.noreply.github.com>

Author: Sunrisepeak

tests/e2e/340_no_orphan_survives_a_killed_mcpp.sh

@@ -33,12 +33,23 @@ work="$(mktemp -d)"; trap 'rm -rf "$work"' EXIT
 cd "$work"
 mkdir -p src
 
-# Counting by executable name, not by a `pgrep -f` pattern: this script's own
-# command line contains the word, and a pattern match finds itself.
+# COUNTING BY EXECUTABLE NAME AND BY WORKING DIRECTORY, AND BOTH HALVES MATTER.
+#
+# Not by a `pgrep -f` pattern: this script's own command line contains the word,
+# and a pattern match finds itself.
+#
+# And not every ninja on the machine: a build server, a developer's second
+# checkout, or another test running in parallel can start one inside this
+# check's window, and a baseline taken before the build does not cover a process
+# that appears during it. Measured in an ecosystem sandbox — which shares the
+# host's PID namespace — where an unrelated session's ninja in a different
+# project failed this assertion. The predicate was right and the object was
+# wrong.
 count_ninja() {
   local n=0 p
   for p in /proc/[0-9]*; do
-    [[ "$(cat "$p/comm" 2>/dev/null)" == "ninja" ]] && n=$((n+1))
+    [[ "$(cat "$p/comm" 2>/dev/null)" == "ninja" ]] || continue
+    case "$(readlink "$p/cwd" 2>/dev/null)" in "$work"*) n=$((n+1));; esac
   done
   echo "$n"
 }