fix: correct 7 bugs found in security audit

Critical:
- crypto.rs (server): use BTreeMap for canonical JSON so key order
  matches JS client's JSON.stringify sort — sig verification was always
  failing silently
- heartbeat.js: capture sentSalt before rotating to next_salt so both
  sides compute next_chain_hash with the same salt — chain was broken
  after the first heartbeat

High:
- trust.rs: avg_speed = total_dist / total_time_ms (px/ms), not
  distance/event_count — legitimate users were always rejected
- ratelimit.rs: add evict_stale() to drain expired entries and prevent
  unbounded HashMap memory growth
- cleanup.rs: call rl.evict_stale() from the periodic cleanup loop

Medium:
- vm.rs: op 0x08 (NOT) is unary — split from binary-op arm so depth
  is not decremented incorrectly
- wasm/crypto.rs: replace unwrap() panics in sign_message /
  get_public_key with unwrap_or_default(); add JS-side guard

Low:
- shared/Cargo.toml, wasm/Cargo.toml: remove leftover add-this comments
- Dockerfile: rust:1.88-bookworm -> rust:1.87-bookworm (1.88 non-existent)

server/src/crypto.rs

@@ -1,5 +1,6 @@
-use ed25519_dalek::{VerifyingKey, Signature};
+use ed25519_dalek::{Signature, VerifyingKey};
 use shared::protocol::HeartbeatRequest;
+use std::collections::BTreeMap;
 
 pub fn verify_signature(
     pub_key_bytes: &[u8],
@@ -11,15 +12,16 @@ pub fn verify_signature(
     let sig_bytes = hex::decode(&req.signature)?;
     let sig = Signature::from_slice(&sig_bytes)?;
 
-    // Build canonical JSON exactly as client signed (sorted keys, no extra spaces)
-    let payload = serde_json::json!({
-        "sessionId": req.session_id,
-        "prevHash": req.prev_hash,
-        "timestamp": req.timestamp,
-        "entropyData": req.entropy_data,
-        "stackState": req.stack_state,
-        "fingerprint": req.fingerprint,
-    });
+    // Build canonical JSON with BTreeMap so keys are sorted alphabetically,
+    // matching the JS client's JSON.stringify(obj, Object.keys(obj).sort()).
+    // Sorted order: entropyData, fingerprint, prevHash, sessionId, stackState, timestamp
+    let mut payload: BTreeMap<&str, serde_json::Value> = BTreeMap::new();
+    payload.insert("entropyData", serde_json::to_value(&req.entropy_data)?);
+    payload.insert("fingerprint", serde_json::to_value(&req.fingerprint)?);
+    payload.insert("prevHash", serde_json::json!(req.prev_hash));
+    payload.insert("sessionId", serde_json::json!(req.session_id));
+    payload.insert("stackState", serde_json::to_value(&req.stack_state)?);
+    payload.insert("timestamp", serde_json::json!(req.timestamp));
     let message = serde_json::to_string(&payload)?;
 
     pk.verify_strict(message.as_bytes(), &sig)?;