perf(scripting): cut out-of-process runner host CPU; compile timeout from compile start; quiet start-failure logs (lmxopcua#789, #790, #788)

#789 (partial, further tuning deferred): combining inline frame writer, buffered
frame reader, one timer per evaluation for T and T+G, slot release on the reader
before the caller resumes, a non-spinning per-connection result dispatcher, and
blocking pipe I/O on Unix (both ends) so the socket never enters the socket
engine; TerminateJobObject kill on Windows. macOS host CPU at 10k evals/s K=2:
228-240 % -> 79 % (InProcess 69 %, IPC share +10 vs budget 25); windev 153 % ->
123 % (+115, still a miss). Harness reports the "vs InProcess" share, warm-cache
worker RSS, and times kill latency from the soft-timeout answer.

#790: the worker sends LoadStarted; the compile timeout is armed then, so a load
queued behind other compiles is never killed or struck.

#788: one Error with stack per down period, rate-limited Warnings with attempt
count, one Information on recovery.

Design doc §8.2 notes + §12.2 record.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/Core/ZB.MOM.WW.OtOpcUa.Core.Scripting.Protocol/Protos/script_worker.proto

@@ -26,6 +26,7 @@ message Envelope {
     DiagnosticsRequest diag_request = 21;  // host -> worker (tests / ops)
     DiagnosticsReply diag_reply = 22;      // worker -> host
     Cancel cancel = 23;                    // host -> worker, at the soft deadline
+    LoadStarted load_started = 24;         // worker -> host, when a compile thread picks a load up (lmxopcua#790)
   }
 }
 
@@ -39,6 +40,7 @@ message Hello {
   uint32 max_log_lines = 7;         // outbound log buffer cap
   string pe_cache_dir = 8;          // the node's compiled-PE cache (design §7); empty = no cache
   bytes pe_cache_key = 9;           // its HMAC key; the worker loads only entries that verify under it
+  uint32 compile_threads = 10;      // dedicated compile threads; 0 = the worker's default (min(8, max(1, CPUs/2)))
 }
 
 message Ready {
@@ -81,6 +83,13 @@ enum PeCache {
   PE_CACHE_REJECTED = 3;            // an entry failed verification, was deleted, and the source was compiled
 }
 
+// A compile thread has taken the load off the worker's queue and is about to compile (or PE-load) it. The host's load
+// timeout runs from here, not from when the host sent the load: a load queued behind other compiles is not stuck and
+// must not be blamed for the queue (lmxopcua#790).
+message LoadStarted {
+  uint64 load_id = 1;
+}
+
 message LoadResult {
   uint64 load_id = 1;
   repeated LoadOutcome outcomes = 2;