longTauGnssCoupling: engine wiring for the coast-cap (--coast-cap, default off)

[bobvan]

Summary

Feeds the scheduler-side coast-cap (#84) with real inputs so the adaptive coast is bounded by the DO's stochastic wander on hardware — the tau~64–256 s hump fix end-to-end. Behind --coast-cap, default off.

Stacked on #84 (scheduler API). Runtime-depends on #83 (honest Q[3,3]) for the p22 path.

Three parts:

• do_state.derive_coast_tdev_from_char — log-log power-law fit of the DO's characterized freerun TDEV curve (tdev_ns_by_tau_s) → (tdev_ref_ns, tdev_slope, 1.0) for coast_cap_from_tdev.
• DOFreqEst.project_p22_coast(max_tau) — predict-only P[2,2] over the coast as a precomputed table (one O(max_tau) projection/epoch; the coast_cap_from_p22 closure looks it up — addresses Charlie's perf nit). Honest only with Q-from-char (qFromCharPerActuator: derive Q[3,3] (sigma_do_freq) from char ADEV #83).
• engine — when --coast-cap: derive coast_tdev from the DO char → DisciplineScheduler ctor; each epoch build the p22_at_tau closure from project_p22_coast and pass it to compute_adaptive_interval. --coast-cap-k-sigma sets the confidence factor.

Scope

• Wires the coast-cap (tdev + p22). The graded taper (distance_to_lock) is deferred to the disciplineModeFsm work, which defines the convergence metric + thresholds.
• Only the main DOFreqEst servo path; the CM_TDC path is unchanged.
• Default-off / byte-identical when --coast-cap absent.

Test plan

• 11 new tests (TDEV power-law fit incl. WFM/RWFM/skips/source-preference; project_p22_coast quadratic-from-constant-freq, RWFM super-linear, length/index0/zero-τ). Full suite 1428 passed. Both flags verified in --help.
• closedLoopServoSim A/B (Charlie) on honest Q[3,3] (qFromCharPerActuator: derive Q[3,3] (sigma_do_freq) from char ADEV #83): --coast-cap on vs off — confirm the tau~64–256 s hump drops and the >60 s coast no longer diverges.
• Lab A/B on PiFace/clkPoC3 (have freerun chars) after the sim validates (Main coordinates).

Merge order: #83 + #80 → #84 → this → sim A/B → lab.

🤖 Generated with Claude Code

[bobvan]

Superseded by #86 (longTau stack squashed into one PR per Bob, 2026-05-29). Engine wiring is included there. Closing tip-first to avoid the stacked-PR auto-close gotcha.