Add encode/decode for the Google Encoded Polyline format

[gistrec]

Summary

Ports PolyUtil.encode/decode (Google Encoded Polyline Algorithm Format) from android-maps-utils — first of the upstream-parity features planned for 1.1.0 (next: simplify + is_closed_polygon, which will reuse decode in tests).

• New header <geo/encoding.hpp> — keeps <geo/poly.hpp> free of <string>/<vector>. Pulled in by the <geo/geo.hpp> umbrella; picked up automatically by the CMake install and the build2 hxx{**} glob.
• geo::encode(const Path&) → std::string — same Path contract as the other container functions; coordinates quantized to 1e-5° (~1 m).
• geo::decode(std::string_view) → std::vector<LatLng> — bounds-checked: a string truncated mid-point yields the points decoded so far and drops the incomplete trailing point (the Java original throws IndexOutOfBounds). Malformed input is memory-safe but yields unspecified coordinates (the format has no checksum).

Portability notes

• The Java/Kotlin implementation relies on wrapping two's-complement int arithmetic; left-shifting a negative signed value is UB in C++17, so the zig-zag/varint accumulation is done in unsigned types — bit-identical to upstream on well-formed input.
• r >> 1 on a negative value (arithmetic shift) is implementation-defined in C++17 but guaranteed on every supported compiler (GCC/Clang/MSVC) and standardized in C++20; noted in a comment.

Docs

• New "Polyline encoding" section in docs/api.md with the reference example; Conventions updated (include strategy, std::bad_alloc note) and encode added to the Path list.
• README: new feature bullet, header count/size refreshed.
• No overlap with the api.md hunks of open PRs Document distance_to_segment limits; fix epsilon doc and test helper #26/Normalize offset/offset_origin output longitude to [-180, 180) #27/Document edge-case behavior in api.md #31.

Test plan

• Encoding.encode: byte-for-byte match with the reference strings from the Encoded Polyline Algorithm Format docs (_p~iF~ps|U_ulLnnqC_mqNvxq@, ?~oia@), quantization collapse.
• Encoding.decode: reference string, empty input, two truncation variants (mid-chunk and between chunks).
• Encoding.encode_decode_roundtrip: poles, antimeridian, sub-quantum coordinates (error ≤ 5e-6°).
• Full suite: 37/37 locally; clean under -Wall -Wextra -Wpedantic -Wconversion -Werror.
• CI matrix (gcc/clang/macos/windows + install/consumer + build2).