docs(battlenode): document four latent low-tier hygiene hazards

Comment-only; behavior-preserving; 231 BattleNode tests green.

- OutboundSequencer._archive: name the unbounded-per-match growth + ack-prune point.
- NodeCrypto.BuildAes: SECURITY remarks on key-derived IV reuse + base64 entropy loss;
  warn against caching the session key.
- MatchContext/BattlePlayer: FOOTGUN notes on reference-based record equality over the deck list.
- RecordTokensFrom: TRUST note on isSelf/idx overwrite; name the idx>deckCount guard for
  untrusted peers (not added — trusted-LAN today).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Bridge/BattlePlayer.cs

@@ -1,5 +1,10 @@
 namespace SVSim.BattleNode.Bridge;
 
 /// <summary>One player slot for a pending battle. Carries the viewer's identity and
-/// the per-battle MatchContext snapshot built at do_matching time.</summary>
+/// the per-battle MatchContext snapshot built at do_matching time.
+/// <para>FOOTGUN: this is a <c>record</c>, but <see cref="Context"/> transitively holds an
+/// <c>IReadOnlyList&lt;long&gt;</c> (the deck), so the synthesized value-equality is REFERENCE-based
+/// on that list — two BattlePlayers with equal deck *contents* compare unequal. Don't use
+/// BattlePlayer / <see cref="MatchContext"/> as dictionary keys or <c>Distinct()</c> / <c>HashSet</c>
+/// operands without first giving them content equality. Not exercised today.</para></summary>
 public sealed record BattlePlayer(long ViewerId, MatchContext Context);

SVSim.BattleNode/Bridge/MatchContext.cs

@@ -5,6 +5,9 @@ namespace SVSim.BattleNode.Bridge;
 /// server-authored frame lifecycle on WS connect. SVSim.BattleNode does not know how to build this — the HTTP-side
 /// per-mode controller is the source. Snapshot semantics: cosmetic changes between matching
 /// and WS connect have no effect on the in-battle render.
+/// <para>FOOTGUN: as a record holding <see cref="SelfDeckCardIds"/> (an IReadOnlyList), the
+/// synthesized value-equality is reference-based on that list — see <see cref="BattlePlayer"/>.
+/// Don't use as a dict key / <c>Distinct()</c> operand without content equality.</para>
 /// </summary>
 public sealed record MatchContext(
     // Player's drafted deck — exactly 30 entries, idx 1..30 paired with the chosen cardIds