routedQErrArm: per-edge chi2 router for the TICC arm (flag, default off)

[bobvan]

Summary

Implements the agreed routedQErrArm design. Each TICC edge routes (priority-ordered) to one of three candidates based on chi²; first to pass the gate wins:

Candidate	model	H	R
external	z = ticc_diff + ext_qerr, h = −x[2]	[0,0,−1,0]	R_base
internal	h = −x[2] − qerr(x[0]) (today)	[−1,0,−1,0]	R_base + R_lin
raw	h = −x[2]	[0,0,−1,0]	R_base + tick²/12

chi² is the correlation-quality detector: a mis-correlated external qErr is wrong by ~a tick = conspicuous outlier vs the small external R → falls through to raw. An F10T's uncorrelated qErr always fails → routes to raw, no per-receiver flag.

Design points (from the dayplan review)

• Priority-ordered, not argmin-chi²: raw has the largest R so it would always win a min-chi² contest. External is tried first (most informative) and accepted only if consistent.
• Raw R = sawtooth variance (tick²/12 = 5.33 ns²), not the worst-case half-tick (impl-flag-1).
• Reuses the existing recv_mono matching (qVIR=165x) — qerr_for_ticc_pps_ns is already computed at the servo epoch; this just delivers it to Arm 4 (impl-flag-2).
• External uses H=[0,0,−1,0] — clean x[2]-only update, no x[0] coupling (matters on --no-ppp-arm where x[0] is loose).

Coupling note

Tick discrimination needs the honest small Q from qFromCharPerActuator. With the 0.92 ns default, Q re-inflates P[2,2] to σ≈0.93 ns each predict, where a full-tick (8 ns) mismatch is only ~8.6σ (χ²≈74 < 100) and would be wrongly accepted. The router operates in the char-Q regime — documented in the tests.

Test coverage (7 new, 1245 total)

• well-correlated external fires; mis-correlated (tick-off) falls through; no-external → internal/raw; F10T random qErr → mostly raw; disabled → always internal; raw R = sawtooth variance; external doesn't couple x[0].

Test plan

• 1245/1245 pass; default-off byte-identical
• Closed-loop A/B in Charlie's closedLoopServoSim (PiFace clean-correlation + MadHat F10T auto-reject)
• Lab A/B on PiFace + MadHat after the sim validates

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