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>

run.sh

@@ -0,0 +1,5 @@
+ python main.py \
+    --video1 1.mp4 \
+    --video2 2.mp4 \
+    --output_dir ./my_scene \
+    --fps 2