refactor: move planner headers into planning/planners/

[zkingston]

Stacked on #119 (custom-nn) — base branch is custom-nn, so the diff here is just the single top commit.

Pure refactor, no functional change. Moves the planner headers under planning/planners/:

• planning/{rrtc,rrtc_settings,prm,fcit,grrtstar,grrtstar_settings,aorrtc,aorrtc_settings,roadmap}.hh → planning/planners/
• Rewrites include paths across the ~15 dependents (Python/WASM bindings, rrtc_example.cc, planner.hh, phs.hh, utils.hh).

Verified: builds clean, core library + all static robot headers + Python bindings.