Feat v0.7.12 batch.exec concurrent execution primitive

Add batch.exec(callables, timeout=10) — a general-purpose concurrent
execution primitive that runs arbitrary Starlark callables in parallel,
each in its own thread with independent step budget. Enables extensions
to fan out library function calls (frozen exports from lib.require)
without decomposing them back into raw http.post parameters.

New permission: batch.exec. Max 8 callables, timeout 1-30s.
Nesting prohibited via atomic flag. 12 new tests, all pass with -race.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/models/models_extension_perm.go

@@ -36,6 +36,7 @@ const (
 	ExtPermConnectionsRead      = "connections.read"
 	ExtPermTriggersRegister     = "triggers.register"
 	ExtPermRealtimePublish      = "realtime.publish"
+	ExtPermBatchExec            = "batch.exec"
 )
 
 // ValidExtensionPermissions is the set of recognized permission keys.
@@ -50,6 +51,7 @@ var ValidExtensionPermissions = map[string]bool{
 	ExtPermConnectionsRead:      true,
 	ExtPermTriggersRegister:     true,
 	ExtPermRealtimePublish:      true,
+	ExtPermBatchExec:            true,
 }
 
 // ── Extension Permission Model ───────────────