feat: self-host configure generation for stupidtools

Wire the full pipeline into the binary: parse + validate the KDL buildfile,
discover extensions, bridge Lua, detect the C toolchain, and emit ./configure.
Adds the project's own stupid.kdl build file and a temp-dir-isolated self-host
integration test proving the generated configure + make rebuild the tool with
-std=c23. tests/run.sh tests 6+8 re-pin the interim 'accepted, no read' CLI
assertions to the wired 'missing buildfile -> exit 1' behavior.

tests/run.sh

@@ -98,11 +98,13 @@ else
     fail "no arguments exited rc=$noargs_rc (want 1): $noargs_out"
 fi
 
-# --- 6. one positional buildfile must be accepted (exit 0, no read) ------
-if "$BIN" buildfile.kdl >/dev/null 2>&1; then
-    pass "one positional buildfile is accepted (exit 0)"
+# --- 6. a missing buildfile is a runtime error (exit 1) -------------------
+"$BIN" buildfile.kdl >/dev/null 2>&1
+rc=$?
+if [ "$rc" -eq 1 ]; then
+    pass "missing buildfile exits 1 (runtime error)"
 else
-    fail "one positional buildfile was rejected (rc=$?)"
+    fail "missing buildfile exited rc=$rc (want 1)"
 fi
 
 # --- 7. --version=1 is an unknown option form -> exit 2 -------------------
@@ -115,10 +117,12 @@ else
 fi
 
 # --- 8. -- ends option parsing; following text is positional --------------
-if "$BIN" -- --bogus >/dev/null 2>&1; then
-    pass "-- terminates option parsing (--bogus taken as buildfile)"
+"$BIN" -- --bogus >/dev/null 2>&1
+rc=$?
+if [ "$rc" -eq 1 ]; then
+    pass "-- terminates option parsing (--bogus taken as a missing buildfile: rc=1, not 2)"
 else
-    fail "-- --bogus exited non-zero (rc=$?)"
+    fail "-- --bogus exited rc=$rc (want 1, proving --bogus was a buildfile not an option)"
 fi
 
 # --- 9. two positionals are a usage error -> exit 2 -----------------------