docs: S0 draft — S1 execution guard for moving bodies before S2

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/moving_bodies.md

@@ -310,9 +310,18 @@ recorded as not runnable rather than replaced by another test.
 - Motion-record host layout and upload path; exclusion of moving bodies from
   the packed `CurvedBoundaryKernel` links (stationary packing unchanged).
 - Explicit rejections for unsupported modes.
+- S1 execution guard: until the S2 moving kernels exist, any `run(n)` with
+  `n > 0` (and `step(n)`) while a moving body is declared raises before any
+  device mutation (no action/motion upload, no launch), including the
+  nominal at-rest case, because the moving body is excluded from the static
+  packed links and nothing else would apply its boundary. `run(0)` keeps its
+  existing behaviour. Simulations without moving bodies are unaffected.
+  Preparation/commit helpers are exercised by host tests with mocks; no
+  operational unsupported stepping path is exposed.
 - Host tests (Astra runs them): rejection-before-mutation, band geometry,
   derived centres, fail-closed behaviour under an injected upload/sync
-  failure, stationary packing byte-identical. No kernels, no GPU runs, no
+  failure, stationary packing byte-identical, execution guard (positive-step
+  run with a moving body raises before upload/launch; stationary unaffected). No kernels, no GPU runs, no
   y-only VIV oscillator (S4), no force RHS.
 
 ## 11. Remaining decisions