docs: align README and API docs with current routing/spatial implementation

[blackopsrepl]

Problem

The documentation currently overstates or misstates some implementation details:

• README says "R-Tree Spatial Indexing" but the code uses K-D trees.
• README suggests A*/Dijkstra routing, but current public route calls pass a zero heuristic, which behaves like Dijkstra.
• README promises "Full road-following geometries", while the main route APIs still snap to nearest nodes.

Why this matters

This creates user confusion and makes correctness/performance expectations unclear.

Scope

• Update README feature list and API examples.
• Clarify the difference between node-snapped and edge-snapped routing if both remain public.
• Document current complexity honestly until the implementation changes.

Acceptance criteria

• README terminology matches the implementation.
• API docs explicitly describe which public methods are node-snapped vs edge-snapped.
• Performance claims are accurate and non-ambiguous.