solve_global_scene: treat MP_MAXITER as calibration failure

[rocketmark]

Summary

solve_global_scene uses res <= 0 to detect solver failure, but MP_MAXITER (return code 5) is not ≤ 0. When the GSS solver hits its iteration limit without converging, status_failure is false, the unconverged lighthouse positions are accepted, and they are written to disk.

On the next launch those positions are loaded as the starting point for calibration. If the unconverged solve is significantly wrong, all subsequent tracking is corrupted and the only recovery is to delete config.json.

Demonstration

BEFORE fix:
GSS solver hits iteration limit
res = MP_MAXITER (5)
res <= 0 → false
status_failure = false
Unconverged lighthouse positions written to disk
Next launch: loads bad positions → tracking corrupted
Result: config.json must be deleted to recover

AFTER fix:
GSS solver hits iteration limit
res = MP_MAXITER (5)
res <= 0 || res == MP_MAXITER → true
status_failure = true
GSS result discarded, existing positions preserved
Result: calibration retried on next GSS cycle

The regular per-object pose solver (line ~522) already rejects res <= 0 correctly; solve_global_scene was missing the MP_MAXITER case.

Impact

Any deployment that triggers GSS calibration (lighthouse position solving) is exposed. The failure mode is silent. No crash, no warning, but tracking quality degrades permanently until config is cleared. More likely on resource-constrained hardware (embedded, Pi) where the solver may not converge within the default iteration budget.

Change

One line in src/poser_mpfit.c, inside solve_global_scene only. The per-object pose solver is not touched.

Found via

Observed in production: GSS calibration would occasionally produce wildly wrong lighthouse positions that persisted across restarts. Correlating with solver return codes showed MP_MAXITER returns were being silently accepted. Deleting config.json recovered tracking; adding res == MP_MAXITER to the failure condition prevented recurrence.

[bl4ckb0ne]

Missing Assisted-by tag to the commit message. You can also drop the Fix: part.

[rocketmark]

Changes made.