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>
Master key never leaves WASM linear memory. Held in Zeroizing<[u8;32]>
inside a thread_local HashMap keyed by u32. lock() removes + zeroizes.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
