Allocation-free trajectory core

The first capability cluster of the motion umbrella — the pure algorithmic layer taktora-motion-core.

Feature: Allocation-free trajectory core FEAT_0091
status: open
satisfies: FEAT_0090
is refined by: ADR_0099

taktora-motion-core — the pure algorithmic layer. Commanded axis setpoints are computed as bounded, allocation-free, panic-free functions of (dt, master). The crate owns no I/O, no threads, and no shared mutable state, and carries no dependency on the executor or iceoryx2, so it is host-testable with no runtime and its temporal and information-exchange independence (for the diverse-monitor safety argument) can be reasoned about without the soft-RT machinery.

The Motion generators — Idle, Velocity (also the virtual master), Trapezoid, jerk-limited SCurve, Gear (electronic gearing), FlyingSaw (quintic catch-up / synchronous / return), and Cam (&'static quintic table) — are ticked through a fixed-capacity AxisGroup in a build-time topological order (masters before slaves) so coupling is same-cycle coherent. Superimposed corrective motion (PLCopen MC_MoveSuperimposed) is realised as an additive jerk-limited overlay on the Axis itself rather than a generator arm — a Motion cannot hold a Motion without heap indirection. The SCurve profile is cross-checked against the Ruckig time-optimal oracle; on-the-fly retargeting from a non-zero state (Ruckig online) remains a deferred follow-on.