Add ResampleConcat; harden Resample against non-monotonic reference clocks

[cboulay]

Summary

Combining two acquisition streams that share a nominal rate but drift slightly (e.g. two hubs at ~30 kHz with 0.1–0.5% effective-rate mismatch) is done by resampling one stream onto the other's clock and concatenating along the channel axis. The existing Resample → Merge graph has two failure modes when the reference clock misbehaves:

1. Seizing. A backward jump in the reference clock leaves every incoming reference value behind the resampler's high-water mark, so it stops emitting and the whole graph hangs.
2. Silent misalignment. When the resampler holds back or drops reference samples, the raw reference stream feeding MERGE.INPUT_SIGNAL_A is no longer sample-aligned with the resampled stream — Align does a one-time offset alignment then reads in lockstep, so it concatenates mismatched samples.

Changes

resample.py

• output_reference setting + new OUTPUT_REFERENCE stream: buffers the reference data and emits it gathered onto the exact grid the signal was resampled onto, so a downstream Concat can combine the two with no second time-alignment (aligned by construction).
• reference_reset_after_chunks (default 3): detects a sustained backward clock jump and re-anchors the high-water mark with a RuntimeWarning instead of seizing. Self-correcting jitter is dropped, keeping output monotonic. Set to inf to restore the previous behaviour.
• push_reference CoordinateAxis fix: no longer unconditionally reads ax.gain (which crashed on a per-sample-timestamp reference).

resampleconcat.py (new)

• ResampleConcat fuses ResampleProcessor (output_reference=True) and ConcatProcessor into a single linear two-input unit — no Align, no diamond graph. Reuses all interpolation logic from resample.py and all axis/attr-merge logic from concat.py; only the orchestration is new.

Tests

• tests/unit/test_resample.py: CoordinateAxis reference, output_reference grid-alignment, reset recovery, and reset-disabled-stalls.
• tests/unit/test_resampleconcat.py (new): two-rate combine with numeric provenance check, no-signal, reset recovery.
• tests/integration/ezmsg/test_resample_merge_system.py (new): healthy graph; seize when recovery disabled (documents the original bug); recovery via OUTPUT_REFERENCE; composite recovery.

Full suite: 3596 passed, 5 skipped locally.

Notes

Output across a genuine reference clock reset is necessarily discontinuous — the hardening is a guardrail (don't hang, warn loudly) rather than a substitute for sanitising reference timestamps upstream.