M1a: survival core, pause menu, title screen and save/load

Design: docs/SURVIVAL_DESIGN.md (core loop, systems, architecture, phases).

Survival core (M1a):
- Items/recipes as data (data/*.json) in the addon SDK's define_item /
  define_recipe shape; procedural item meshes and offscreen-rendered icons
- Inventory class (40 slots, hotbar = row 1) shared by player and chests;
  click / right-click split / shift-click quick-move
- Actions autoload: the intent layer (craft, pickup, harvest_hit, place,
  transfer) that validates every change; becomes the RPC surface in M2
- Gathering: ground pickups (sticks, stones, flint); trees chopped with an
  axe fall and leave stumps; boulders mined with a pickaxe shrink per hit
- Crafting screen with station checks (workbench); placement mode with
  green/red ghost, rotation and overlap/slope validation
- Pieces: campfire (light, flames, embers), workbench, chest (16 slots)
- HUD: hotbar, held tool view model with swing, prompts, toasts

Menus and saving:
- ESC pause menu: resume, controls, fullscreen, mouse sensitivity, quit
- Title screen over a flyover of the island: Continue / New World (name +
  seed) / Load World (with delete) / Quit
- Saves autoload: one folder per world under user://worlds, atomic writes
  with a .bak; stores only player changes (player, pieces, chest contents,
  harvested resources, taken pickups) and regenerates the rest from the seed
- Autosave every 2 min, Save & Quit to title/desktop, save on window close

Fixes: UI screens now fill the viewport (were pinned top-left); title
backdrop no longer captures the mouse.

Tests: test_survival_core (31 checks), test_save_load (round trip + slots),
test_terrain_edges now checks 8 seeds for deep pits / unbanked water.

scripts/building/campfire.gd

@@ -0,0 +1,99 @@
+extends "res://scripts/building/piece.gd"
+## Campfire: flickering light, flames, embers and smoke. A "campfire" station
+## (cooking arrives in M1b along with fuel and warmth).
+
+var _light: OmniLight3D
+var _t := 0.0
+var _noise := FastNoiseLite.new()
+
+func _build() -> void:
+	add_to_group("station:campfire")
+	_noise.frequency = 2.5
+	_light = OmniLight3D.new()
+	_light.light_color = Color(1.0, 0.62, 0.3)
+	_light.light_energy = 2.2
+	_light.omni_range = 11.0
+	_light.omni_attenuation = 1.4
+	_light.shadow_enabled = true
+	_light.position = Vector3(0, 0.7, 0)
+	add_child(_light)
+
+	add_child(_particles("Flames", 26, 0.55, Vector3(0, 0.12, 0), 0.5, 1.0, 0.5, 0.9, _gradient([
+		[0.0, Color(1.0, 0.8, 0.35, 0.0)], [0.15, Color(1.0, 0.62, 0.2, 0.75)],
+		[0.6, Color(0.95, 0.3, 0.06, 0.45)], [1.0, Color(0.5, 0.08, 0.02, 0.0)]]), true, 0.32, 0.12))
+	add_child(_particles("Embers", 10, 2.0, Vector3(0, 0.3, 0), 0.8, 2.0, 0.5, 1.0, _gradient([
+		[0.0, Color(1.0, 0.75, 0.35, 1.0)], [1.0, Color(1.0, 0.3, 0.05, 0.0)]]), true, 0.04, 0.15))
+	var smoke := _particles("Smoke", 10, 5.0, Vector3(0, 1.0, 0), 0.6, 1.1, 0.6, 1.4, _gradient([
+		[0.0, Color(0.55, 0.54, 0.52, 0.0)], [0.25, Color(0.58, 0.57, 0.55, 0.07)],
+		[1.0, Color(0.7, 0.7, 0.7, 0.0)]]), false, 0.9, 0.15)
+	smoke.spread = 25.0
+	smoke.gravity = Vector3(0.35, 0.25, 0.1)   # drifts with the wind
+	# TODO(M1b): smoke renders as an opaque dark ball whatever its ramp colour
+	# (alpha-blended unshaded particles; additive flames are fine). Disabled
+	# until that's fixed, likely by moving to a small particle shader.
+	smoke.queue_free()
+
+func display_name() -> String:
+	return "Campfire"
+
+func _process(delta: float) -> void:
+	_t += delta
+	var f := _noise.get_noise_1d(_t * 10.0)
+	_light.light_energy = 2.2 + f * 0.7
+	_light.position = Vector3(f * 0.05, 0.7 + f * 0.05, 0)
+
+static func _gradient(stops: Array) -> Gradient:
+	var g := Gradient.new()
+	g.offsets = PackedFloat32Array(stops.map(func(s): return s[0]))
+	g.colors = PackedColorArray(stops.map(func(s): return s[1]))
+	return g
+
+static func _soft_dot() -> GradientTexture2D:
+	var g := Gradient.new()
+	g.offsets = PackedFloat32Array([0.0, 1.0])
+	g.colors = PackedColorArray([Color(1, 1, 1, 1), Color(1, 1, 1, 0)])
+	var t := GradientTexture2D.new()
+	t.gradient = g
+	t.fill = GradientTexture2D.FILL_RADIAL
+	t.fill_from = Vector2(0.5, 0.5)
+	t.fill_to = Vector2(0.5, 0.0)
+	t.width = 64
+	t.height = 64
+	return t
+
+func _particles(label: String, amount: int, lifetime: float, pos: Vector3, vmin: float, vmax: float,
+		smin: float, smax: float, ramp: Gradient, additive: bool, quad_size: float, radius: float) -> CPUParticles3D:
+	var p := CPUParticles3D.new()
+	p.name = label
+	p.amount = amount
+	p.lifetime = lifetime
+	p.position = pos
+	p.direction = Vector3.UP
+	p.spread = 12.0
+	p.initial_velocity_min = vmin
+	p.initial_velocity_max = vmax
+	p.gravity = Vector3(0.15, 0.4, 0.05)
+	p.emission_shape = CPUParticles3D.EMISSION_SHAPE_SPHERE
+	p.emission_sphere_radius = radius
+	p.scale_amount_min = smin
+	p.scale_amount_max = smax
+	p.color_ramp = ramp
+	# Flames shrink as they rise.
+	var curve := Curve.new()
+	curve.add_point(Vector2(0.0, 1.0))
+	curve.add_point(Vector2(1.0, 0.35 if additive else 1.6))
+	p.scale_amount_curve = curve
+	var quad := QuadMesh.new()
+	quad.size = Vector2.ONE * quad_size
+	var m := StandardMaterial3D.new()
+	m.shading_mode = BaseMaterial3D.SHADING_MODE_UNSHADED
+	m.billboard_mode = BaseMaterial3D.BILLBOARD_PARTICLES
+	m.vertex_color_use_as_albedo = true
+	m.transparency = BaseMaterial3D.TRANSPARENCY_ALPHA
+	m.blend_mode = BaseMaterial3D.BLEND_MODE_ADD if additive else BaseMaterial3D.BLEND_MODE_MIX
+	m.albedo_texture = _soft_dot()
+	if additive:
+		m.albedo_color = Color(1.5, 1.5, 1.5)   # mildly HDR so the glow picks it up
+	quad.material = m
+	p.mesh = quad
+	return p