# T3 — Roadblocks and manned checkpoints

Read `docs/living-island/README.md` first, including the rules section. Depends on T1, T2.

## Goal

Make the road network feel controlled. A checkpoint is **permanent evidence**
(barriers, a parked truck, sandbags, a light at night) plus a **conditional
garrison** (2–4 soldiers, materialised only when someone is near). The evidence
costs no CPU and is always there; the soldiers cost agents and are not.

This is the single biggest "living island" win per CPU cycle: the player sees
occupied terrain everywhere and pays for soldiers only where they are.

## What to build

1. **Site kind.** Extend `ERR_LocationKind` with `CHECKPOINT`, and
   `ERR_SiteDefinition` with the fields a checkpoint needs: prop layout choice,
   garrison size, whether it blocks the road, and the approach direction.
   Update `ERR_SiteDefinition` validation and its existing tests accordingly.

2. **Static layout.** `addons/core/Scripts/Game/ERR/ERR_CheckpointLayout.c`
   places vanilla props relative to the site origin and a road heading: barriers
   angled across one lane, sandbag positions, a parked vehicle, a barrel. Keep
   it to **one authored arrangement with a mirrored variant** — do not build a
   procedural composition system.
   - Use only vanilla prefabs. Every `{GUID}path` you add must be present in
     `docs/vanilla-resources.json`, or `tools/check.py` will reject it. Find real
     GUIDs by searching the installed vanilla data, not by guessing.
   - Props are spawned once during preparation and never ticked.

3. **Checkpoint records.** Each checkpoint registers a `CHECKPOINT` record with
   `ERR_ForceDuty.HOLD`, importance `NORMAL`, and a small garrison. The record's
   position is the checkpoint; it does not move.

4. **Authored content.** Add 6–10 checkpoints to
   `addons/core/Configs/ERR/EveronSites.conf` on the main road junctions between
   the prison, the radio station and the helicopter site — the routes players
   actually travel. Pick junctions, bridges and town entrances.

5. **Behaviour when manned.** The garrison gets a `DefendWaypoint` at the
   checkpoint. One soldier stands at the barrier, the rest hold nearby positions.
   Idle activity comes from T4 — leave hooks, do not implement smart actions here.

## Acceptance criteria

1. Driving the prison → radio → helipad route, the player passes at least three
   checkpoints, each visibly built and manned when approached.
2. With no players near any checkpoint, checkpoint garrisons contribute **zero**
   active agents.
3. Props survive their garrison being wiped — a cleared checkpoint still looks
   like a checkpoint, and the record is marked eliminated so it does not repopulate.
4. Checkpoint props never block the road so completely that a vanilla car cannot
   pass; leave a drivable gap or a moveable barrier.
5. `python3 tools/check.py` passes, including the resource-GUID rules.

## Tests to write

`tests/test_checkpoint_layout.py`:

- The pure placement function returns positions that are within the expected
  radius of the site origin and correctly rotated for a given road heading.
- The mirrored variant is the mirror image, and the drivable gap is preserved in
  both variants (assert a minimum clear width along the road axis).
- `ERR_SiteDefinition` validation rejects a checkpoint with zero garrison or a
  garrison above the allowed maximum.

## Do not

- Do not import any composition, prefab or layout from the original mod.
- Do not spawn checkpoint props per-approach; they are placed once at preparation.
- Do not exceed 12 checkpoints on the first pass. Authoring more is cheap in
  props but adds up in records and in preparation time.