feat: improve SAMVG local path fitting

[rasros]

Improves the native SAMVG path without external renderers or scientific-stack dependencies.

• Adds bounded spatial fill groups and unified fill/stroke local fitting backed by the analytic CUDA renderer.
• Implements the dissertation-style two-phase recovery benchmark and saved galleries.
• Adds hybrid strokes, OCR text verification, and removes SciPy/scikit-learn from the SAMVG extra.
• Speeds SAM inference with FP16, batched prompts, CPU mask post-processing, and shared encoder embeddings.
• Speeds impact filtering with incremental scoring and dependency-free scanline connected components/hole processing.

Validation: SAMVG tests, filled-path/fidelity tests, Ruff, and Pyrefly.

Generated benchmark outputs under bench/results/ are intentionally not committed.