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/sim/BuildingSystem.h

@@ -170,6 +170,7 @@ private:
     const ModuleDef*    findModuleDef(const std::string& id) const;
     void                initBuffers(Building& b, const RecipeDef& recipe) const;
     void                initShipyardBuffers(Building& b) const;
+    void                initSalvageBayBuffer(Building& b) const;
     std::vector<Port>   computeInputPorts(const Building& b) const;
     std::vector<Item>   rollReprocessingOutput(const RecipeDef& recipe);
     bool                bodyCellsWithinWorldBounds(