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>

docs/STARLARK-REFERENCE.md

@@ -242,6 +242,39 @@ instances = workflow.list_instances(workflow_id, status="active")
 # Returns list of instance dicts
 ```
 
+### batch
+
+**Permission:** `batch.exec`
+
+Run multiple callables concurrently. Each callable gets its own
+execution thread with an independent step budget.
+
+```python
+jira = lib.require("jira-client")
+confluence = lib.require("confluence-client")
+
+results, errors = batch.exec([
+    lambda: jira.create_issue(issue_data),
+    lambda: confluence.create_page(page_data),
+    lambda: send_notification(user_id),
+], timeout=15)
+
+# results[i] = return value of callables[i], or None on error
+# errors[i]  = None on success, or error string on failure
+# All three ran concurrently.
+```
+
+**Constraints:**
+
+- Max 8 callables per call. Dispatched concurrently via goroutines.
+- `timeout` (optional): 1–30 seconds per branch (default 10).
+- `lib.require()` is not available inside branch callables.
+  Load libraries before the `batch.exec` call.
+- `batch.exec()` cannot be called from within a branch (no nesting).
+- `print()` output from branches is discarded.
+
+---
+
 ## Example: automated stage hook
 
 A simple hook that reads a setting, queries data, and advances: