[feat]: major refactor and feature updates for C API, offset pipeline, and coverage

[NorthBoundWisdom]

This PR includes a large-scale refactor and feature update set.

1. Expand and stabilize C API surface:

• Add polyline helpers (invert, prune singularities, convert arcs, normalize)
• Add point containment API and tolerance get/set/reset API
• Add spatial index and offset-loop topology APIs

2. Upgrade parallel offset options and behavior:

• Introduce structured options (join type, end cap type, miter limit)
• Add miter/bevel join support including arc-involved joins
• Add square/butt end cap handling for open polylines

3. Improve robustness and reliability:

• Guard tolerance config for concurrent reads/writes
• Apply miter-limit validation only for miter join mode
• Harden edge cases in intersection and offset clipping paths

4. Strengthen tests and benchmarks:

• Add TEST_cavc_api_regression and TEST_cavc_offset_islands
• Extend matrix/stress regressions for join/end-cap/topology scenarios
• Add offset-islands benchmarks and CMake wiring updates

5. Changelog and project metadata updates:

• Refresh changelog entries for feature scope and semantics
• Keep behavior/documentation aligned with implemented options

Please note: this is intentionally a substantial refactor + feature batch to align the library/API with current geometry and regression coverage goals.