Fix Salvage Bay drop-off: config-driven output-buffer capacity

Salvage ships flew to their assigned Salvage Bay with full cargo and
waited there forever without delivering. A Salvage Bay's
outputBuffer.capacity was never initialized (stayed 0), so
deliverScrapToSalvageBay always rejected the hand-over, cargo never
drained, and DeliverScrap stayed the winning behavior indefinitely.

Give the bay a config-driven output-buffer capacity:
- BuildingDef gains optional outputBufferCapacity; ConfigLoader parses
  the new output_buffer_capacity key.
- salvage_bay in both buildings.toml files sets it to 20.
- initSalvageBayBuffer applies it on both operational-creation paths
  (construction completion and placeImmediate).
- REQ-BLD-SALVAGE-BAY documents the config-sized buffer and hand-over.
- Tests: config parse assertion and end-to-end delivery hand-over.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VGNNLeFWhVzvxkK9qVXP2K

bin/test/data/config/buildings.toml

@@ -83,6 +83,7 @@ id = "salvage_bay"
 cost = 25
 player_placeable = true
 construction_time_seconds = 15
+output_buffer_capacity = 20
 surface_mask = [
   "SAA",
   "SAA>",