test(e2e): 118 asserts the depfile degradation on Windows instead of a capability it lacks

Windows plus a GNU-dialect toolchain — which is what the CI leg's clang is —
genuinely cannot track textual includes: the depfile GCC emits for a module
TU needs an awk filter to be loadable by ninja, and native Windows has no
awk. #257 does not fix that platform; it makes the engine SAY so through
diag::degraded, and the Windows CI log carries that message 13 times.

Dropping the `# requires: gcc` gate pointed the test at that platform for
the first time, where it asserted a rebuild that cannot happen. Asserting
the capability there would be wrong, and re-gating the test to POSIX would
put the gap back out of sight.

So on Windows the test now asserts the degradation IS reported and stops.
That is the stronger contract: silence is the actual defect, and this pins
it. Everywhere else the rebuild assertions are unchanged.

Author: Sunrisepeak

tests/e2e/118_purview_include_rebuild.sh

@@ -24,6 +24,19 @@ subst() {  # subst <sed-expr> <file>
     sed "$1" "$2" > "$2.tmp" && mv "$2.tmp" "$2"
 }
 
+# Windows + a GNU-dialect toolchain (the CI leg's clang) is the one
+# combination that genuinely CANNOT track textual includes: the depfile GCC
+# emits for a module TU needs an awk filter to be loadable by ninja, and
+# native Windows has no awk. #257 does not fix that — it makes the engine SAY
+# so, through diag::degraded. On that platform this test therefore asserts
+# the degradation is reported rather than asserting a capability the build
+# does not have; silence would be the actual defect.
+case "$(uname -s)" in
+    MINGW*|MSYS*|CYGWIN*) EXPECT_TRACKING=0 ;;
+    *)                    EXPECT_TRACKING=1 ;;
+esac
+DEGRADED_MSG="emits no GNU depfile"
+
 TMP=$(mktemp -d)
 trap "rm -rf $TMP" EXIT
 
@@ -55,9 +68,22 @@ name    = "purviewinc"
 version = "0.1.0"
 EOF
 
-out="$("$MCPP" run 2>&1 | tail -1)"
+run_log=$("$MCPP" run 2>&1)
+out="$(echo "$run_log" | tail -1)"
 [[ "$out" == "41" ]] || { echo "unexpected initial output: $out"; exit 1; }
 
+if [[ $EXPECT_TRACKING -eq 0 ]]; then
+    echo "$run_log" | grep -q "$DEGRADED_MSG" || {
+        echo "$run_log"
+        echo "FAIL: this toolchain/platform cannot emit a depfile, and said nothing."
+        echo "      A capability gap must be reported, not silent (#257)."
+        exit 1
+    }
+    echo "  windows: depfile degradation reported as expected; rebuild tracking not asserted"
+    echo "OK"
+    exit 0
+fi
+
 subst 's/41/42/' src/vals.inc
 
 out="$("$MCPP" run 2>&1 | tail -1)"