chore: low-severity sweep — import session cap, full compile-error lists, leaf-name-fallback validation warning

#05-T24. Three low-severity cleanups, each TDD'd:
- TransportOptions.MaxConcurrentImportSessions (8): BundleSessionStore.Open rejects
  a new session past the cap (evicting expired entries first), bounding the N×~200 MB
  decrypted-content footprint. Soft cap (count/add not atomic).
- ScriptCompiler.TryCompile now joins ALL forbidden-API violations / compile errors
  (string.Join instead of [0]) so an operator sees every problem at once; the T15
  verdict cache stores the joined string.
- SemanticValidator emits an advisory warning when a CallScript target resolves
  ONLY via the composed leaf-name fallback (dynamic child path, not statically
  verified) instead of silently accepting it.

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

src/ZB.MOM.WW.ScadaBridge.Transport/Import/BundleSessionStore.cs

@@ -73,6 +73,26 @@ public sealed class BundleSessionStore : IBundleSessionStore
     public BundleSession Open(BundleSession session)
     {
         ArgumentNullException.ThrowIfNull(session);
+        // #05-T24: bound the number of concurrently-open sessions — each pins a
+        // decrypted bundle (up to MaxBundleSizeMb of plaintext) in memory until
+        // it expires or is applied/cancelled. Re-opening an existing id (overwrite)
+        // never grows the count, so only a genuinely-new session is gated. A soft
+        // cap: the count/add pair is not atomic, so a race may briefly exceed it
+        // by one — acceptable for a memory-pressure guard, not a security invariant.
+        if (!_sessions.ContainsKey(session.SessionId))
+        {
+            var cap = _options.Value.MaxConcurrentImportSessions;
+            if (_sessions.Count >= cap)
+            {
+                // A full store may be full only of stale entries — reclaim first.
+                EvictExpired();
+                if (_sessions.Count >= cap)
+                {
+                    throw new InvalidOperationException(
+                        $"Too many concurrent import sessions ({cap}). Complete or cancel an in-progress import before starting another.");
+                }
+            }
+        }
         // Overwrite on collision is defensive: GUIDs are random so practical
         // collisions don't happen, but if a caller reuses an id we always
         // honor the latest Open call.

src/ZB.MOM.WW.ScadaBridge.Transport/TransportOptions.cs

@@ -28,6 +28,13 @@ public sealed class TransportOptions
     /// generous headroom for unusually compressible bundles.
     /// </summary>
     public int MaxBundleEntryCompressionRatio { get; set; } = 50;
+    /// <summary>
+    /// #05-T24: maximum number of concurrently-open import sessions. Each open
+    /// session pins a fully-decrypted bundle (up to <see cref="MaxBundleSizeMb"/>
+    /// of plaintext) in memory until it expires or is applied/cancelled, so this
+    /// bounds the N×~200 MB decrypted-content footprint the central node can hold.
+    /// </summary>
+    public int MaxConcurrentImportSessions { get; set; } = 8;
     /// <summary>Gets or sets the maximum number of failed passphrase unlock attempts before a session is locked.</summary>
     public int MaxUnlockAttemptsPerSession { get; set; } = 3;
     /// <summary>Gets or sets the maximum number of unlock attempts allowed per IP address per hour.</summary>