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.

stupid.kdl

@@ -0,0 +1,79 @@
+/* stupid.kdl - stupidtools' OWN build file (todo 23: self-host).
+ *
+ * The project describes itself in its own DSL. This is a sibling of
+ * tests/fixtures/stupid.kdl (the todo-9 grammar fixture): richer, with the
+ * REAL source list, so `src/stupidtools stupid.kdl` generates a ./configure
+ * that can rebuild the tool end to end. The autotools path (configure.ac +
+ * Makefile.am) remains the bootstrap; this file is what the self-host test
+ * (tests/selfhost.sh) drives.
+ *
+ * DSL grammar (src/kdl/schema.h): `project` first, then targets/features/
+ * options. The `target` source list is descriptive in v1 (the generated
+ * configure substitutes @VAR@ into a user-provided Makefile.in; it does not
+ * generate a Makefile) but lists every translation unit so the self-host
+ * Makefile.in can compile them all. */
+
+project "stupidtools" version "1.0.0"
+
+// The POSIX surface the tool itself relies on (fork/exec/pipe/readdir for
+// detection and discovery). A real check: the generated configure probes
+// unistd.h with the detected C compiler.
+feature "posix" {
+    header "unistd.h"
+}
+
+// The one artifact: every translation unit of the tool, plus the posix
+// feature it needs at build time.
+target "default" {
+    src "src/main.c"
+    src "src/cli.c"
+    src "src/error.c"
+    src "src/span.c"
+    src "src/kdl/lexer.c"
+    src "src/kdl/parser.c"
+    src "src/kdl/value.c"
+    src "src/kdl/schema.c"
+    src "src/detect/check_registry.c"
+    src "src/detect/checks.c"
+    src "src/detect/probe.c"
+    src "src/detect/resolve.c"
+    src "src/gen/sh_emit.c"
+    src "src/gen/configure.c"
+    src "src/gen/config.c"
+    src "src/gen/args.c"
+    src "src/ext/discovery.c"
+    src "src/ext/lua.c"
+    src "src/ext/abi.c"
+    src "src/ext/api.c"
+    src "src/ext/lang_c.c"
+    src "src/ext/lang_cpp.c"
+    src "thirdparty/lua/lapi.c"
+    src "thirdparty/lua/lauxlib.c"
+    src "thirdparty/lua/lbaselib.c"
+    src "thirdparty/lua/lcode.c"
+    src "thirdparty/lua/lctype.c"
+    src "thirdparty/lua/ldebug.c"
+    src "thirdparty/lua/ldo.c"
+    src "thirdparty/lua/ldump.c"
+    src "thirdparty/lua/lfunc.c"
+    src "thirdparty/lua/lgc.c"
+    src "thirdparty/lua/llex.c"
+    src "thirdparty/lua/lmem.c"
+    src "thirdparty/lua/lobject.c"
+    src "thirdparty/lua/lopcodes.c"
+    src "thirdparty/lua/lparser.c"
+    src "thirdparty/lua/lstate.c"
+    src "thirdparty/lua/lstring.c"
+    src "thirdparty/lua/ltable.c"
+    src "thirdparty/lua/ltm.c"
+    src "thirdparty/lua/lundump.c"
+    src "thirdparty/lua/lvm.c"
+    src "thirdparty/lua/lzio.c"
+    src "thirdparty/lua/lstrlib.c"
+    src "thirdparty/lua/ltablib.c"
+    feature "posix"
+}
+
+// --enable-debug / --disable-debug (default off), like configure.ac's
+// release/debug split.
+option "debug" default=#false