fix(wasm): impl Drop for SessionHandle clears registry entry

Closes the P1.1 defense-in-depth gap: wasm-bindgen's auto-generated
.free() previously dropped the SessionHandle wrapper (a u32) without
removing the SESSIONS HashMap entry, leaving the master key and
image_secret in WASM linear memory until JS explicitly called
lock(handle). Drop now wires .free() to session::remove, and the
new native test pins the contract.

Refs: docs/superpowers/specs/2026-05-04-security-polish-design.md (Phase 1)
Refs: docs/superpowers/reviews/2026-05-04-architecture-review.md (P1.1)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-wasm/src/lib.rs

@@ -12,7 +12,13 @@ use zeroize::Zeroizing;
 
 use relicario_core::{derive_master_key, imgsecret, KdfParams};
 
-/// Handle type returned from `unlock`. Backed by a `u32`; opaque to JS.
+/// Handle returned from `unlock`. Backed by a `u32`; opaque to JS.
+///
+/// Dropping the handle (or invoking `.free()` from JS) removes the entry from
+/// the session registry, zeroizing the wrapped master key and image_secret.
+/// `lock(handle)` remains available as the explicit early-cleanup path; the
+/// `Drop` impl is the safety net that catches code paths which forget to call
+/// `lock` before letting the handle go out of scope.
 #[wasm_bindgen]
 pub struct SessionHandle(u32);
 
@@ -22,6 +28,23 @@ impl SessionHandle {
     pub fn value(&self) -> u32 { self.0 }
 }
 
+impl Drop for SessionHandle {
+    fn drop(&mut self) { let _ = session::remove(self.0); }
+}
+
+#[doc(hidden)]
+pub fn __test_make_handle() -> SessionHandle {
+    SessionHandle(session::insert(
+        Zeroizing::new([0x77u8; 32]),
+        Zeroizing::new([0u8; 32]),
+    ))
+}
+
+#[doc(hidden)]
+pub fn __test_session_exists(handle: u32) -> bool {
+    session::with(handle, |_| ()).is_some()
+}
+
 #[wasm_bindgen]
 pub fn unlock(
     passphrase: &str,
@@ -533,6 +556,19 @@ mod session_tests {
         assert!(!session::remove(h));   // second remove false
     }
 
+    #[test]
+    fn dropping_session_handle_clears_registry_entry() {
+        session::clear();
+        let handle = SessionHandle(session::insert(
+            Zeroizing::new([0x33u8; 32]),
+            Zeroizing::new([0u8; 32]),
+        ));
+        let id = handle.value();
+        assert!(session::with(id, |_| ()).is_some());
+        drop(handle);
+        assert!(session::with(id, |_| ()).is_none());
+    }
+
     #[test]
     fn with_yields_key_only_while_session_lives() {
         session::clear();