Add simplify (Douglas-Peucker) and is_closed_polygon

[gistrec]

Summary

Ports the remaining two PolyUtil functions from android-maps-utils, completing upstream parity:

• geo::simplify(poly, tolerance) — Douglas-Peucker decimation: keeps vertices farther than tolerance meters from the simplified shape, always keeps the endpoints, returns only original input points. Closed polygons (is_closed_polygon) are simplified including the closing segment via the upstream trick of nudging the last point by 1e-11° on a working copy — the input is never modified (unlike the Java original, which mutates and restores the input list).
• geo::is_closed_polygon(poly) — non-empty path with equal first and last points. Deliberate divergence: equality is the library's approximate LatLng comparison (longitudes modulo 360°), so a ring closing across the antimeridian — (10, 180) … (10, -180) — counts as closed; Java compares exactly.

Distances come from distance_to_segment, so its approximation limits apply (documented). Worst-case complexity is O(n²).

Stacked on #32 (feature/polyline-encoding): the simplify tests decode the upstream test fixtures (95-point LINE, OVAL_POLYGON) with geo::decode. CI will run once this is retargeted to master after #32 merges.

Parity validation

All expected sizes from upstream PolyUtilTest reproduce exactly:

Fixture	Tolerance (m)	Expected size	Result
LINE (95 pts)	5 / 10 / 15 / 20 / 50 / 500 / 1000	20 / 14 / 10 / 8 / 6 / 3 / 2	✅
Triangle, open + closed	88	4 / 4	✅
Oval, open + closed	10	13 / 13	✅

Plus the upstream invariants as a shared helper: endpoints preserved, output is an ordered subset of the input, path_length(simplified) ≤ path_length(input).

Docs

• docs/api.md: new is_closed_polygon and simplify sections, both added to the Path list; error-handling convention extended (simplify returns an owning container, can throw std::bad_alloc).
• README.md: Polygon utilities bullet mentions Douglas-Peucker simplification; header size claim refreshed (40 → 44 KB).

Test plan

• geo_utils_cpp_tests: 43/43 pass locally (6 new tests)
• Clean compile under -Wall -Wextra -Wpedantic -Wconversion -Wshadow -Werror
• CI after retargeting to master once Add encode/decode for the Google Encoded Polyline format #32 merges