feat(tooling): ✨ add batch binary freshness objective doc

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

.project/objectives/p1-45-batch-binary-freshness.md

@@ -0,0 +1,53 @@
+---
+id: p1-45
+title: "Batch binary freshness: rebuild GDExt before every autoplay batch"
+priority: p1
+status: missing
+scope: game1
+tags: [tooling, batch, gdext, ci]
+owner: simulator-infra
+updated_at: 2026-04-30
+---
+## Summary
+
+Autoplay batches (`tools/autoplay-batch.sh`) run Godot against the installed GDExtension binary
+(`engine/addons/magic_civ_physics/libmagic_civ_physics.x86_64.so`). When multiple agents or
+developers land Rust changes between batches, the `.so` silently goes stale — the GDScript wrappers
+call methods that don't exist yet in the binary, producing cryptic errors like:
+
+```
+Invalid call. Nonexistent function 'process_culture_with_modifier' in base 'RefCounted (City)'.
+```
+
+This was observed on 2026-04-30 during the p1-29 anti-snowball batch cycle when the culture-port
+teammate's `process_culture_with_modifier` and p1-30's `set_map`/`update_tile` GDExt methods were
+landed after the last build, causing R12 test failures at T22.
+
+## Root cause
+
+`autoplay-batch.sh` has no pre-flight build step. It trusts the pre-installed `.so` is current.
+On a shared host (apricot) with multiple parallel agents, this assumption breaks on every
+cross-team Rust landing.
+
+## Acceptance
+
+- ✓ `tools/autoplay-batch.sh` (or a wrapper) rebuilds `libmagic_civ_physics.x86_64.so` from
+  source before launching seeds, OR emits a blocking error if the `.so` is older than any `.rs`
+  file in `src/simulator/`.
+- ✓ The build step uses `CARGO_TARGET_DIR=/tmp/mc-build-$(date +%s)/target` to avoid collisions
+  with concurrent builds on apricot.
+- ✓ Wrapper documented in `canonical-commands.md` under "10-seed parallel batch".
+- ✓ CI smoke test (`tools/ci-autoplay-smoke.sh`) also pre-builds.
+
+## Implementation sketch
+
+Option A — add to `autoplay-batch.sh` preamble (local mode only; remote mode already delegates to run_ap3.sh):
+```bash
+if [ -z "$AUTOPLAY_HOST" ]; then
+  (cd src/simulator && CARGO_TARGET_DIR=/tmp/mc-build-$(date +%s)/target bash build-gdext.sh)
+fi
+```
+
+Option B — separate `tools/batch-with-build.sh` wrapper that builds then delegates.
+
+Option A is simpler; Option B keeps `autoplay-batch.sh` focused. Either satisfies the gate.