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/tests/session_drop.rs

@@ -0,0 +1,16 @@
+//! Belt-and-suspenders companion to the native `dropping_session_handle_clears_registry_entry`
+//! test in `lib.rs`. This file exists for `wasm-pack test --node` symmetry; the
+//! native test in the same crate is what gates CI.
+
+use wasm_bindgen_test::wasm_bindgen_test;
+
+use relicario_wasm::{__test_make_handle, __test_session_exists};
+
+#[wasm_bindgen_test]
+fn dropping_session_handle_clears_registry_entry() {
+    let handle = __test_make_handle();
+    let id = handle.value();
+    assert!(__test_session_exists(id));
+    drop(handle);
+    assert!(!__test_session_exists(id));
+}