Add two-video drone 3DGS pipeline with Apple Silicon fixes

- main.py: extract frames from two videos, run COLMAP feature extraction
- match_features.py: Python-based within-video SIFT matching via OpenCV
  (replaces colmap exhaustive_matcher which segfaults on ARM64 in COLMAP 4.x)
- match_crossvideo.py: exhaustive cross-video matching (v1×v2) to stitch
  two flights into a single COLMAP model
- run.sh: entry point for frame extraction + feature extraction
- train_splat.sh: ns-process-data → splatfacto → .ply export, with
  correct PATH for Homebrew ffmpeg and MPS device flags for Apple Silicon
- .gitignore: exclude videos, generated scene data, venv, logs
- README.md: full pipeline walkthrough, all known issues and fixes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,24 @@
+# Videos (too large for git)
+*.mp4
+*.mov
+*.avi
+
+# Generated scene data
+my_scene/
+outputs/
+
+# Python environment
+venv/
+__pycache__/
+*.pyc
+*.pyo
+
+# Logs
+*.log
+my_scene_build.log
+
+# macOS
+.DS_Store
+
+# Editor
+.claude/