Add building ghost preview

AGRARIAN_DEVELOPMENT_ROADMAP.md

@@ -610,7 +610,9 @@ Target deliverable: A small group can join a server, spawn into one biome, gathe
   shelter through the server-authoritative placement component. Fully modular
   wall-by-wall building stays deferred to `0.2.E Permanent Structures`.
 - [x] Create build placement mode.
-- [~] Add ghost preview.
+- [x] Add ghost preview. The building placement component now exposes preview
+  validity/transform state and draws an assetless green/red wireframe ghost
+  footprint at the snapped placement location for MVP development builds.
 - [x] Add placement validation.
 - [x] Add basic shelter piece.
 - [ ] Add wall piece if needed.

Docs/TechnicalDesignDocument.md

@@ -86,6 +86,11 @@ Early runtime systems should remain small and explicit:
   as inventory parts and then combined into a single placeable primitive
   shelter actor. Fully modular wall-by-wall construction is deferred to
   permanent structures so the first survival loop remains reliable.
+- `UAgrarianBuildingPlacementComponent` owns the MVP placement preview. It
+  traces from the player view, snaps to the configured grid, validates distance
+  and collision, broadcasts Blueprint-readable preview state, and draws a
+  green/red wireframe ghost footprint in development builds so placement can be
+  tested before final mesh/material ghost assets exist.
 - Persistence layer: save/load contracts for player and world state.
 
 Blueprints can compose and expose these systems, but core replicated behavior

Scripts/verify_building_ghost_preview.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python3
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+HEADER = ROOT / "Source" / "AgrarianGame" / "AgrarianBuildingPlacementComponent.h"
+SOURCE = ROOT / "Source" / "AgrarianGame" / "AgrarianBuildingPlacementComponent.cpp"
+ROADMAP = ROOT / "AGRARIAN_DEVELOPMENT_ROADMAP.md"
+TECHNICAL_DESIGN = ROOT / "Docs" / "TechnicalDesignDocument.md"
+
+
+def compact(path: Path) -> str:
+    return " ".join(path.read_text(encoding="utf-8").split())
+
+
+def require(path: Path, text: str) -> None:
+    data = compact(path)
+    if text not in data:
+        raise SystemExit(f"FAIL: {path.relative_to(ROOT)} missing required text: {text}")
+
+
+def main() -> None:
+    require(HEADER, "FAgrarianBuildPreviewUpdatedSignature")
+    require(HEADER, "OnBuildPreviewUpdated")
+    require(HEADER, "bShowGhostPreview")
+    require(HEADER, "GetPlacementPreviewState")
+    require(HEADER, "TickComponent")
+    require(SOURCE, "DrawDebugBox")
+    require(SOURCE, "ValidGhostPreviewColor")
+    require(SOURCE, "InvalidGhostPreviewColor")
+    require(SOURCE, "OnBuildPreviewUpdated.Broadcast")
+    require(ROADMAP, "[x] Add ghost preview.")
+    require(TECHNICAL_DESIGN, "green/red wireframe ghost footprint")
+    print("PASS: building ghost preview support is present.")
+
+
+if __name__ == "__main__":
+    main()

Source/AgrarianGame/AgrarianBuildingPlacementComponent.cpp

@@ -2,13 +2,14 @@
 
 #include "AgrarianBuildingPlacementComponent.h"
 #include "AgrarianInventoryComponent.h"
+#include "DrawDebugHelpers.h"
 #include "GameFramework/Pawn.h"
 #include "GameFramework/Controller.h"
 #include "Engine/World.h"
 
 UAgrarianBuildingPlacementComponent::UAgrarianBuildingPlacementComponent()
 {
-	PrimaryComponentTick.bCanEverTick = false;
+	PrimaryComponentTick.bCanEverTick = true;
 	SetIsReplicatedByDefault(true);
 }
 
@@ -55,6 +56,16 @@ bool UAgrarianBuildingPlacementComponent::GetPlacementPreview(FTransform& OutTra
 	return true;
 }
 
+bool UAgrarianBuildingPlacementComponent::GetPlacementPreviewState(FTransform& OutTransform, FText& FailureReason) const
+{
+	if (!GetPlacementPreview(OutTransform, FailureReason))
+	{
+		return false;
+	}
+
+	return CanPlaceAtTransform(ActiveBuildClass, OutTransform, FailureReason);
+}
+
 bool UAgrarianBuildingPlacementComponent::CanPlaceAtTransform(TSubclassOf<AActor> BuildClass, const FTransform& PlacementTransform, FText& FailureReason) const
 {
 	if (!BuildClass)
@@ -148,6 +159,38 @@ void UAgrarianBuildingPlacementComponent::ServerPlaceBuildable_Implementation(TS
 	OnBuildPlaced.Broadcast(PlacedActor);
 }
 
+void UAgrarianBuildingPlacementComponent::TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction)
+{
+	Super::TickComponent(DeltaTime, TickType, ThisTickFunction);
+
+	if (!bShowGhostPreview || !ActiveBuildClass || !GetWorld())
+	{
+		return;
+	}
+
+	FTransform PreviewTransform;
+	FText FailureReason;
+	const bool bCanPlace = GetPlacementPreviewState(PreviewTransform, FailureReason);
+	const FColor PreviewColor = bCanPlace ? ValidGhostPreviewColor : InvalidGhostPreviewColor;
+	const FVector PreviewExtent(
+		FMath::Max(PlacementProbeRadius, 25.0f),
+		FMath::Max(PlacementProbeRadius, 25.0f),
+		FMath::Max(PlacementProbeRadius * 0.5f, 25.0f));
+
+	DrawDebugBox(
+		GetWorld(),
+		PreviewTransform.GetLocation() + FVector(0.0f, 0.0f, PreviewExtent.Z),
+		PreviewExtent,
+		PreviewTransform.GetRotation(),
+		PreviewColor,
+		false,
+		GhostPreviewLifetimeSeconds,
+		0,
+		GhostPreviewLineThickness);
+
+	OnBuildPreviewUpdated.Broadcast(bCanPlace, PreviewTransform, FailureReason);
+}
+
 bool UAgrarianBuildingPlacementComponent::HasPlacementCost(FText& FailureReason) const
 {
 	const UAgrarianInventoryComponent* Inventory = GetOwner() ? GetOwner()->FindComponentByClass<UAgrarianInventoryComponent>() : nullptr;

Source/AgrarianGame/AgrarianBuildingPlacementComponent.h

@@ -9,6 +9,7 @@
 
 DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FAgrarianBuildPlacedSignature, AActor*, PlacedActor);
 DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FAgrarianBuildPlacementFailedSignature, FText, Reason);
+DECLARE_DYNAMIC_MULTICAST_DELEGATE_ThreeParams(FAgrarianBuildPreviewUpdatedSignature, bool, bCanPlace, FTransform, PreviewTransform, FText, FailureReason);
 
 UCLASS(ClassGroup = (Agrarian), BlueprintType, Blueprintable, meta = (BlueprintSpawnableComponent))
 class UAgrarianBuildingPlacementComponent : public UActorComponent
@@ -24,6 +25,9 @@ public:
 	UPROPERTY(BlueprintAssignable, Category = "Agrarian|Building")
 	FAgrarianBuildPlacementFailedSignature OnBuildPlacementFailed;
 
+	UPROPERTY(BlueprintAssignable, Category = "Agrarian|Building")
+	FAgrarianBuildPreviewUpdatedSignature OnBuildPreviewUpdated;
+
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Agrarian|Building")
 	TSubclassOf<AActor> ActiveBuildClass;
 
@@ -45,12 +49,30 @@ public:
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Agrarian|Building", meta = (EditCondition = "bSnapToGrid", ClampMin = "1"))
 	float GridSize = 50.0f;
 
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Agrarian|Building|Preview")
+	bool bShowGhostPreview = true;
+
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Agrarian|Building|Preview", meta = (ClampMin = "0"))
+	float GhostPreviewLifetimeSeconds = 0.0f;
+
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Agrarian|Building|Preview", meta = (ClampMin = "0"))
+	float GhostPreviewLineThickness = 2.0f;
+
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Agrarian|Building|Preview")
+	FColor ValidGhostPreviewColor = FColor(70, 220, 120);
+
+	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Agrarian|Building|Preview")
+	FColor InvalidGhostPreviewColor = FColor(230, 80, 65);
+
 	UFUNCTION(BlueprintCallable, Category = "Agrarian|Building")
 	void SetActiveBuildable(TSubclassOf<AActor> BuildClass, const TArray<FAgrarianItemStack>& Cost);
 
 	UFUNCTION(BlueprintCallable, Category = "Agrarian|Building")
 	bool GetPlacementPreview(FTransform& OutTransform, FText& FailureReason) const;
 
+	UFUNCTION(BlueprintCallable, Category = "Agrarian|Building")
+	bool GetPlacementPreviewState(FTransform& OutTransform, FText& FailureReason) const;
+
 	UFUNCTION(BlueprintCallable, Category = "Agrarian|Building")
 	bool CanPlaceAtTransform(TSubclassOf<AActor> BuildClass, const FTransform& PlacementTransform, FText& FailureReason) const;
 
@@ -60,6 +82,8 @@ public:
 	UFUNCTION(Server, Reliable, BlueprintCallable, Category = "Agrarian|Building")
 	void ServerPlaceBuildable(TSubclassOf<AActor> BuildClass, FTransform PlacementTransform);
 
+	virtual void TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction) override;
+
 protected:
 	bool HasPlacementCost(FText& FailureReason) const;
 	void ConsumePlacementCost();