Fix temp-file leak in loadKey and clear all per-proof caches on dispose

[unp1]

Server resource cleanup (keyext.api):

• Fix RPCLayer handler thread shutdown and unknown-id NPE #8 (real leak) loadKey wrote a json-rpc-*.key file into the system temp dir on every call (loadKey/loadTerm/loadProblem) and never deleted it. The file is now removed in a finally once loading is done.
• Implement treeSubtree; fix websocket stream setup and auto() stdout logging #11 (defensive) ProofContainer.dispose() only cleared mapNode (which is never even populated). It now clears all per-proof caches incl. the printed-sequent cache mapGoalText. Note: the container is normally dropped from mapProof and GC'd on dispose, so this is eager/defensive release rather than a hard leak.

Test: LoadKeyTempFileTest asserts loadTerm leaves no temp file behind (fails on the old code).

Stacked on #7; retarget to main once it merges. Touches KeyApiImpl/KeyIdentifications in different methods than #6, so they merge cleanly.

PR has been created with AI tooling