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

src/lib/config/BuildingsConfig.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -18,6 +19,10 @@ struct BuildingDef
     // Stored as raw strings here; parsing into per-cell tiles + output ports
     // happens when buildings are placed, not at load time.
     std::vector<std::string> surfaceMask;
+
+    // Output-buffer holding size for buildings without a recipe-driven buffer.
+    // Only the Salvage Bay sets this (REQ-BLD-SALVAGE-BAY).
+    std::optional<int> outputBufferCapacity;
 };
 
 struct BuildingsConfig