refactor(executor-core): extract sdk/ module + move json↔dynamic bridge

Hoist the json_to_dynamic / dynamic_to_json helpers out of engine.rs
into a new sdk/bridge.rs so the v1.1.x service modules (KV, docs, …)
can use them without engine.rs being the sole owner. No behavioural
change — the sdk_contract round-trip test pins the observable JSON
fidelity.

Also lands the structural shape that subsequent v1.1.x PRs hook into:

  - sdk::register_all(engine, services, cx) — single per-call hook
    every stateful service registers through. Body is a no-op for
    v1.1.0; SdkCallCx construction inside Engine::execute lands in
    the next commit alongside the new ExecRequest fields it reads.
  - sdk::cx re-exports picloud_shared::SdkCallCx so SDK callers don't
    cross-import shared for one type.

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

crates/executor-core/src/sdk/bridge.rs

@@ -0,0 +1,79 @@
+//! JSON ↔ Rhai `Dynamic` value bridge.
+//!
+//! Originally inline in `engine.rs`; moved here for v1.1.0 so future
+//! service modules (KV in v1.1.1, docs in v1.1.2, …) can convert
+//! values without `engine.rs` being the only owner of the conversions.
+//! Behaviour is unchanged from the pre-extraction implementation —
+//! `sdk_contract.rs::json_round_trip_preserves_nested_shapes` pins the
+//! observable round-trip.
+
+use rhai::{Dynamic, Map};
+use serde_json::Value as Json;
+
+/// Convert a `serde_json::Value` into a Rhai `Dynamic` suitable for
+/// pushing into a script's scope. Numbers prefer the narrowest type
+/// (`i64` over `f64`); anything that can't round-trip falls back to a
+/// string so the script always sees a defined value.
+#[must_use]
+pub fn json_to_dynamic(value: Json) -> Dynamic {
+    match value {
+        Json::Null => Dynamic::UNIT,
+        Json::Bool(b) => b.into(),
+        Json::Number(n) => {
+            if let Some(i) = n.as_i64() {
+                i.into()
+            } else if let Some(f) = n.as_f64() {
+                f.into()
+            } else {
+                n.to_string().into()
+            }
+        }
+        Json::String(s) => s.into(),
+        Json::Array(arr) => arr
+            .into_iter()
+            .map(json_to_dynamic)
+            .collect::<Vec<Dynamic>>()
+            .into(),
+        Json::Object(obj) => {
+            let mut m = Map::new();
+            for (k, v) in obj {
+                m.insert(k.into(), json_to_dynamic(v));
+            }
+            Dynamic::from(m)
+        }
+    }
+}
+
+/// Convert a Rhai `Dynamic` back to a `serde_json::Value`. Custom Rhai
+/// types (timestamps, user-registered modules) fall back to their
+/// `Display` form so they appear as strings in JSON output rather than
+/// failing the response build.
+#[must_use]
+pub fn dynamic_to_json(value: &Dynamic) -> Json {
+    if value.is_unit() {
+        return Json::Null;
+    }
+    if let Ok(b) = value.as_bool() {
+        return Json::Bool(b);
+    }
+    if let Ok(i) = value.as_int() {
+        return Json::Number(i.into());
+    }
+    if let Ok(f) = value.as_float() {
+        return serde_json::Number::from_f64(f).map_or(Json::Null, Json::Number);
+    }
+    if value.is_string() {
+        return Json::String(value.clone().into_string().unwrap_or_default());
+    }
+    if let Some(arr) = value.clone().try_cast::<rhai::Array>() {
+        return Json::Array(arr.iter().map(dynamic_to_json).collect());
+    }
+    if let Some(map) = value.clone().try_cast::<Map>() {
+        let mut out = serde_json::Map::new();
+        for (k, v) in map {
+            out.insert(k.to_string(), dynamic_to_json(&v));
+        }
+        return Json::Object(out);
+    }
+    Json::String(value.to_string())
+}