perf: speed up area preservation with Newton/Steiner buffer search

[wrignj08]

What

Replaces the bracketed Brent's-method search in _preserve_area_with_buffer with a closed-form Steiner-quadratic seed refined by Newton's method.

The buffered area of a polygon follows the Steiner expansion A(d) ≈ A₀ + L·d + π·d² (L = perimeter), so the initial offset is found by solving that quadratic instead of a linear estimate. Each candidate is then refined with Newton's method using the measured boundary length as the derivative — the rate of area change of an outward offset equals its perimeter (coarea identity), so the derivative is essentially exact and free. Because buffered area is monotonic in distance, no bracketing is needed.

This cuts buffer operations per ring from roughly 5–6 to 1–2.

Why

Profiling the full pipeline on examples/Water.gpkg showed area preservation (brentq + repeated full-polygon buffering) was the single largest cost (~34% at 5 iterations), not the Chaikin smoothing itself.

Results (single-core, examples/Water.gpkg, old vs new)

iters	tol	old	new	speedup	buffers old→new	area err old/new	geom Δ med/max
5	0.01	5079 ms	3847 ms	1.32×	2910→801	0.0064% / 0.0064%	0.0003% / 0.0050%
3	0.01	3512 ms	3200 ms	1.10×	2916→869	0.0064% / 0.0064%	0.0002% / 0.0095%
5	0.1	4394 ms	3553 ms	1.24×	2712→781	0.0279% / 0.0279%	0.0011% / 0.0053%

Speedup scales with iteration count and the polygon share of the workload (down to ~1.0× on hole-dominated inputs, where hole-differencing dominates). Verified equivalent across several synthetic datasets (many small, few large, with holes) as well.

Safety

• Output unchanged within the area tolerance: per-polygon symmetric difference ≤ 0.005%, area-preservation accuracy identical.
• Shapes where Newton stalls (pinch-offs, topology changes near the root) fall back to the original Brent's-method search, preserved verbatim as _preserve_area_brentq.
• Pure Python, no new dependencies.

Tests

7 new tests in tests/test_smoothify_core.py (Newton path on concave shapes, the fallback path, a buffer-count regression guard, Newton/brentq agreement, empty input). Full suite: 432 passed. mypy strict + ruff clean.

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