feat: add aerodynamic drag to physics model

[cloudygreybeard]

Summary

• Adds a velocity-dependent drag term to the equation of motion, modelling aerodynamic resistance on the spinning wheel
• The combined deceleration is now dv/dt = -alpha_c - (gamma/m) * v, where alpha_c is the Coulomb term and gamma/m is the drag term
• Resolves the runaway behaviour where a light wheel (e.g. mass=0.1, friction=0.01) would spin for 350+ seconds under pure Coulomb friction
• New --drag flag (default 0.1); set to 0 for pure Coulomb friction
• Delay schedule computed by numerical bisection when drag > 0; closed-form Coulomb expression used when drag = 0
• README extended with sections 2.2 (aerodynamic drag) and updated 2.3 (discrete delay schedule), including equations (5)-(9) and worked examples

Test plan

• All 31 tests pass with race detector
• New tests: drag monotonicity, drag reduces ticks, light wheel stops faster at same v0, drag narrows mass gap, prevents runaway, zero drag falls back to Coulomb, first delay approximates 1/v0
• Smoke tested: mass=0.1 friction=0.01 completes in ~8s (was 350s+)
• go vet and lints clean