Add HMAC-SHA256, base64url, uuid_v4, unix_timestamp, json encode/decode, http auth builtins for soma-license

New builtins in dispatch_builtin:
- hmac_sha256(secret, data) -> String (hex)
- base64_url_encode(s) / base64_url_decode(s) -> String
- unix_timestamp() -> Int
- uuid_v4() -> String (alias for uuid_new)
- json_encode(v) / json_decode(s) -> polymorphic
- json_get_string(obj, key), json_get_int(obj, key), json_get_array(obj, key)
  — work on both Value::Map and Value::Struct (json_parse returns Struct)
- http_get_auth(url, token), http_put_auth(url, token, body), http_delete_auth(url, token)
- string_split_last(s, delim) -> [before, after] on last occurrence
- array_get(arr, idx) -> element

http_serve upgraded to general-purpose router: passes all requests to
handle_request(method, path, body) — not just /axon/message. Method, path,
body stored in GLOBAL_STATE before calling callback; handle_request receives
them as positional args via initial_stack.

env() now returns "" (not Nil) when var is unset — enables `env("X") == ""`
comparisons in Engram code.

run_sub_interpreter_with_stack() added to support pre-pushing args onto the
call stack before entering a function.

Fix pre-existing non-exhaustive match errors in el-fmt, el-types, el-arch
for Retry, Deploy, With, Reason, Parallel, Trace, Activate AST nodes.

Register all builtins in TypeEnv::with_builtins() to eliminate type-checker
warnings for builtin calls.

Cargo.lock

@@ -143,6 +143,15 @@ dependencies = [
  "cpufeatures 0.3.0",
 ]
 
+[[package]]
+name = "block-buffer"
+version = "0.10.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3078c7629b62d3f0439517fa394996acacc5cbc91c5a20d8c658e77abd503a71"
+dependencies = [
+ "generic-array",
+]
+
 [[package]]
 name = "bumpalo"
 version = "3.20.2"
@@ -309,6 +318,17 @@ dependencies = [
  "cipher",
 ]
 
+[[package]]
+name = "digest"
+version = "0.10.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9ed9a281f7bc9b7576e61468ba615a66a5c8cfdff42420a70aa82701a3b1e292"
+dependencies = [
+ "block-buffer",
+ "crypto-common",
+ "subtle",
+]
+
 [[package]]
 name = "displaydoc"
 version = "0.2.5"
@@ -324,6 +344,7 @@ dependencies = [
 name = "el"
 version = "0.1.0"
 dependencies = [
+ "base64",
  "blake3",
  "clap",
  "el-build",
@@ -337,8 +358,11 @@ dependencies = [
  "el-seal",
  "el-test",
  "el-types",
+ "hex",
+ "hmac",
  "reqwest",
  "serde_json",
+ "sha2",
  "thiserror 2.0.18",
  "tiny_http",
  "tokio",
@@ -742,6 +766,21 @@ version = "0.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 
+[[package]]
+name = "hex"
+version = "0.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
+
+[[package]]
+name = "hmac"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6c49c37c09c17a53d937dfbb742eb3a961d65a994e6bcdcf37e7399d0cc8ab5e"
+dependencies = [
+ "digest",
+]
+
 [[package]]
 name = "http"
 version = "1.4.0"
@@ -1616,6 +1655,17 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "sha2"
+version = "0.10.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
+dependencies = [
+ "cfg-if",
+ "cpufeatures 0.2.17",
+ "digest",
+]
+
 [[package]]
 name = "shlex"
 version = "1.3.0"