Feat v0.7.12 batch exec (#66)

Co-authored-by: Jeffrey Smith <jasafpro@gmail.com>
Co-committed-by: Jeffrey Smith <jasafpro@gmail.com>

CHANGELOG.md

@@ -2,6 +2,26 @@
 
 All notable changes to Armature are documented here.
 
+## v0.7.12 — Concurrent Execution Primitive
+
+New `batch` sandbox module. Enables extensions to parallelize arbitrary
+Starlark callables — including frozen library exports from `lib.require()`.
+
+**batch module (permission: `batch.exec`)**
+
+- `batch.exec(callables, timeout=10)` — runs up to 8 zero-arg callables concurrently, each in its own `starlark.Thread` with independent step budget. Returns `(results, errors)` tuple with ordered results. Per-branch timeout (1–30s, default 10).
+- Nested `batch.exec()` calls are prohibited (prevents exponential goroutine growth).
+- `lib.require()` not available inside branches — load libraries before the batch call.
+- `print()` output from branches is discarded.
+
+**Internal**
+
+- New file: `sandbox/batch_module.go`.
+- New permission constant: `ExtPermBatchExec`.
+- Runner wiring: `buildModulesWithLibCtx` creates batch module when permission granted.
+- Design doc: `docs/DESIGN-batch-exec.md`.
+- 12 new tests (parallel ordering, partial failure, timeout, cancellation, cap enforcement, nesting prevention, frozen sharing, permission gating).
+
 ## v0.7.11 — Query & HTTP Ergonomics
 
 Four new Starlark sandbox builtins. No new permissions, no schema changes.