Complete ElkSharp document rendering cleanup and source decomposition

- Fix target-join (edge/4+edge/17): gateway face overflow redirect to left tip
- Fix under-node (edge/14,15,20): push-first corridor reroute instead of top corridor
- Fix boundary-slots (4->0): snap after gateway polish reordering
- Fix gateway corner diagonals (2->0): post-pipeline straightening pass
- Fix gateway interior adjacent: polygon-aware IsInsideNodeShapeInterior
- Fix gateway source face mismatch (2->0): per-edge redirect with lenient validation
- Fix gateway source scoring (5->0): per-edge scoring candidate application
- Fix edge-node crossing (1->0): push horizontal segment above blocking node
- Decompose 7 oversized files (~20K lines) into 55+ partials under 400 lines each
- Archive sprints 004 (document cleanup), 005 (decomposition), 007 (render speed)

All 44+ document-processing artifact assertions pass. Hybrid deterministic mode
documented as recommended path for LeftToRight layouts.

Tests verified: StraightExit 2/2, BoundarySlotOffenders 2/2, HybridDeterministicMode 3/3,
DocumentProcessingWorkflow artifact 1/1.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/workflow/ENGINE.md

@@ -960,6 +960,8 @@ ElkSharp uses a Sugiyama-based layered graph layout with deterministic iterative
 - `LayerSpacing`: horizontal gap between layers (default 60)
 - `Effort`: Draft / Balanced / Best
 
+**Hybrid deterministic mode** is the recommended routing path for `LeftToRight` layouts. It produces zero hard violations on the document-processing reference fixture (44+ geometry assertions). `TopToBottom` remains on the legacy multi-strategy path until explicit parity testing is completed for that direction.
+
 For detailed architecture, see [ElkSharp Rendering Architecture](engine/16-elksharp-rendering-architecture.md).
 
 ### Render Pipeline