feat(stdlib): seven Rhai utility modules + register_stdlib hook

Adds the v1.1.0 user-visible stdlib: regex, random, time, json, base64,
hex, url — each exposed as a `::` namespace mirroring the existing
`log::` pattern. Modules register once at engine build via
`Engine::register_static_module`, distinct from the stateful service
modules (KV, docs, …) that hook into `sdk::register_all` per call.

- regex: linear-time, compile-per-call (no cache by design)
- random: OsRng only; bytes/string capped to prevent script-side blow-up
- time: UTC, ms-since-epoch as canonical i64; RFC 3339 strings for I/O
- json: parse/stringify via existing dynamic<->json bridge
- base64: standard + URL-safe alphabets, Blob and String inputs
- hex: lowercase output, case-insensitive decode
- url: RFC 3986 percent-encoding + encode_query for Maps

Stdlib registration runs unconditionally — including in the parse-only
validate path — so scripts get a uniform surface in both phases.

See docs/sdk-shape.md for the stateless-vs-stateful distinction.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/executor-core/src/engine.rs

@@ -143,6 +143,11 @@ fn build_engine(limits: Limits, logs: Option<Arc<Mutex<Vec<LogEntry>>>>) -> Rhai
         engine.register_static_module("log", build_log_module(logs).into());
     }
 
+    // Stateless utility modules — regex::/random::/time::/json::/base64::/
+    // hex::/url::. Always registered, including in the parse-only validate
+    // path, so script authors get consistent surface in both phases.
+    sdk::stdlib::register_stdlib(&mut engine);
+
     engine
 }