`Drone.Formation` pure planners + tests

[thanos]

Pure module. No processes. No I/O.

Input

%{
  drones: [:a, :b, :c, :d],
  positions: %{a: %{x: -50, y: 0, z: 30, yaw: 0}, ...},
  heading_deg: 0,
  spacing_cm: 100,
  min_separation_cm: 80,
  origin: :centroid
  # origin: {:xy, 0, 0}
  # leader: :a   # optional; plan-time pose only
}

Output

{:ok, %{a: Mission.t(), b: Mission.t(), ...}}
# or
{:error, :separation_violation | :unsupported_formation | :too_few_drones | :leader_unavailable}

Built-in formations

Atom	Min drones	Layout
:front	2	Side-by-side, perpendicular to heading_deg
:column	2	Nose-to-tail along heading_deg
:vee	3	V opening opposite travel (geometry only)
:diamond	4	Four-sided closed layout
:echelon	2	Diagonal stepped rank; opt side: :left | :right
:circle	3	Even spacing on ring; opt radius_cm
:shoulder_pair	2	Alias for :front with two members
:grid	2	Optional classroom row/column lattice

Default reference: :centroid or configured {:xy, x, y} — not a living
leader. Optional leader: uses that member’s pose as the plan-time origin
only. See docs/design/v0_2_0_deferred.md for leader-failure recovery that is
explicitly out of scope.

Each mission should:

1. Move horizontally toward slot (decompose into forward/back/left/right
   relative to current yaw, or rotate then forward — pick one deterministic
   strategy and document it)
2. Optionally hover briefly

Movement strategy for v0.2.0: rotate to face target bearing, move forward
by horizontal distance, restore yaw if needed. Keep altitudes unchanged unless
formation specifies height.

Use existing Drone.Geometry for deltas. Add pure helpers in Drone.Formation
for bearing/distance if needed.

Separation check (Separate, plan-time only)

Before returning missions, verify pairwise horizontal distance between planned
slots >= min_separation_cm. Live Separate / Align / Cohere are deferred.