Gas burner building and the energy resource (#54)

A new building that consumes gas to produce energy, a new resource
stored and moved exactly like ore and gas.

The resource: EnergyResourceGlyph (a cell/bolt mark) + energy's own
warm yellow-green ResourceDefinition, registered in
ResourceDefinitions.ORDER/DEFINITIONS and GameState.RESOURCE_ENERGY.
Inventory/ResourceBar/ResourceTransferList needed no changes - #49
already made them resource-agnostic, confirmed structurally in the
test rather than touched.

The building: GasBurnerBuildingVisual (a hexagonal chamber with intake
vents in gas's colour and output ports in energy's, so the silhouette
itself reads "gas in, energy out"), GasBurnerBuildingDefinition
(capabilities=["gas_burner"], its own build-menu icon appended to
IconButton.Icon rather than inserted, per the ticket's own warning
about renumbering stored scene values), and the matching
UI/BuildMenu/GasBurner* nodes in Main.tscn. Gated behind a new
"Power Systems" skill-tree node, mirroring how "freight" unlocks the
transport hub.

Conversion: _advance_gas_burners(), simulated in GameState alongside
_advance_production/extractor mining so it keeps running while the
player is elsewhere. Withdraws gas and deposits energy in the
building's OWN inventory only - never the bank - so it stalls with no
local gas exactly like a lab waiting on materials.

Logistics: a gas burner's ongoing gas deficit is folded into the
existing queued-building-supply dispatch path (same "needs resource X"
loop production/research orders already use, extended with
gas_burner_gas_deficit()), and a new TO_BUILDING_PICKUP/
TO_MOTHERSHIP_FROM_BUILDING transporter state pair collects its energy
to the bank, mirroring the extractor pickup path per the ticket. The
existing f5c60f8 priority (queued deliveries outrank collection) holds:
the pickup dispatch loop runs last.

Tests: new tests/test_gas_burner.gd (resource genericity, stall
semantics, research gate, and full delivery+pickup logistics via a
real hub), run_tests transporter/resource regressions, and screenshots
of the build menu (all 7 buildings) and transfer menu (all 3
resources). Full suite: 81/81.

Two real bugs found and fixed along the way, neither gas-burner-
specific:
- _transporter_inbound_to() matched ANY non-idle transporter with a
  matching target_id, regardless of purpose. Once pickup states shared
  target_id with delivery states (both address a building, not an
  extractor), a transporter delivering gas made the SAME building look
  "busy" to the unrelated energy-pickup dispatch, starving it even
  with a second, genuinely free transporter sitting idle at the hub.
  Split into _transporter_inbound_to() (delivery states only) and a
  new _transporter_inbound_for_pickup() (pickup states only).
- The build menu's 3-row/6-slot layout (MENU_ROW_TOPS + row content
  sizing) had no room for a 7th building - despite the ticket's own
  note that no layout changes should be needed. A naive 4th row ran
  off the bottom of a 720x1280 viewport; fitting one meant both adding
  a 4th row top AND shrinking the shared button/label/cost sizing so
  each row's content no longer taller than the compressed row spacing.
  Caught only by actually screenshotting it - a real build menu, not
  headless, since --headless renders nothing.

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Main.tscn

@@ -576,6 +576,48 @@ script = ExtResource("19")
 amount = 35
 digit_height = 40.0
 
+[node name="GasBurnerButton" type="Control" parent="UI/BuildMenu"]
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -64
+offset_right = 64
+offset_top = 570
+offset_bottom = 698
+script = ExtResource("10")
+icon = 33
+glow_color = Color(0.95, 0.35, 0.15, 1)
+
+[node name="GasBurnerLabel" type="Label" parent="UI/BuildMenu"]
+mouse_filter = 2
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -150
+offset_right = 150
+offset_top = 704
+offset_bottom = 770
+theme_override_font_sizes/font_size = 32
+autowrap_mode = 2
+text = "Gas Burner"
+horizontal_alignment = 1
+
+[node name="GasBurnerCost" type="Control" parent="UI/BuildMenu"]
+mouse_filter = 2
+anchor_left = 0.5
+anchor_right = 0.5
+anchor_top = 0.5
+anchor_bottom = 0.5
+offset_left = -60
+offset_right = 60
+offset_top = 774
+offset_bottom = 826
+script = ExtResource("19")
+amount = 30
+digit_height = 40.0
+
 [node name="CloseButton" type="Control" parent="UI/BuildMenu"]
 anchor_left = 1.0
 anchor_right = 1.0