runtime: HEAD method dispatches as GET, body suppressed in response

Per RFC 9110 §9.3.2, HEAD must mirror GET headers + Content-Length
without sending a body. Existing http_worker / http_worker_v2 dropped
HEAD straight to the El handler, which had no idea what to do and
returned the catch-all 404 envelope. Link checkers and SEO bots saw
the 404 and reported the site as broken.

Fix layer is in the runtime, not the El handler:

  * http_worker / http_worker_v2 detect HEAD before calling the
    handler, dispatch as method="GET" so handler logic is unchanged,
    record head_only in a thread-local, then call http_send_response.
  * http_send_response reads the thread-local and skips the
    final http_send_all of the body. Status line + headers +
    Content-Length still go out in full.

Verified locally on engram /health: HEAD returns
  HTTP/1.1 200 OK
  Content-Type: application/json; charset=utf-8
  Content-Length: 48
  Connection: close
  (no body — curl reports size_download=0)

compiler.el: rename `target` → `tgt` in main(); the lexer reserves
`target` as a keyword, and the let-binding position requires Ident.
The naming convention was already followed elsewhere in the file
(compile_dispatch's parameter is tgt for exactly this reason); main
was an outlier that the existing Rust-genesis-built elc happened to
parse but bootstrap.py refused, blocking self-host.

el-compiler/src/compiler.el

@@ -172,7 +172,10 @@ fn resolve_imports(src_path: String) -> String {
 //   elc --target=js <source.el> <out.js>       # write JS to file
 fn main() -> Void {
     let argv: [String] = args()
-    let target: String = detect_target(argv)
+    // Use `tgt` not `target`: `target` is a reserved keyword in the lexer
+    // (Section 1.5 of the language spec). detect_target itself is fine
+    // because the function-name position has no token-class restriction.
+    let tgt: String = detect_target(argv)
     let positional: [String] = strip_flags(argv)
     let argc: Int = native_list_len(positional)
     if argc < 1 {
@@ -181,7 +184,7 @@ fn main() -> Void {
     }
     let src_path: String = native_list_get(positional, 0)
     let source: String = resolve_imports(src_path)
-    let out: String = compile_dispatch(target, source)
+    let out: String = compile_dispatch(tgt, source)
     if argc >= 2 {
         let out_path: String = native_list_get(positional, 1)
         let ok: Bool = fs_write(out_path, out)