Add server-side builtins, import system, and http_serve for Neuron Code rewrite

- Import resolution: resolve_imports() pre-processes import statements by
  reading and concatenating referenced .el files before compilation
- http_serve builtin: tiny_http-based server on configurable port; POST
  /axon/message stores request in __request__ state, invokes handle_request
  entry point via sub-interpreter, reads __response__ state for reply
- New builtins: blake3_hash, uuid_new, fs_list_recursive, fs_mkdir, fs_exists,
  path_join, path_parent, str_trim, str_contains, str_replace, str_starts_with,
  str_ends_with, str_last_index_of, json_get, json_array_push, json_array_len,
  now_millis, http_get, http_post, int_to_str
- Catch-all arms in el-types and el-compiler for new AST variants (Import,
  ProtocolDef, ImplDef, Closure, Try, MapLiteral, TypeExpr::Result, TypeExpr::Map)
- Parser: decorators field on FnDef, import/protocol/impl parsing

bin/el/Cargo.toml

@@ -24,3 +24,7 @@ thiserror   = { workspace = true }
 tokio       = { version = "1", features = ["rt", "rt-multi-thread", "macros"] }
 reqwest     = { workspace = true }
 serde_json  = { workspace = true }
+uuid        = { workspace = true }
+walkdir     = { workspace = true }
+tiny_http   = { workspace = true }
+blake3      = { workspace = true }