Split merge module into separate steps: align and concat

[cboulay]

Each of align and concat have utility on their own, and concat has expanded utility including concatenating along a new axis.

AI Summary:

New files:

• src/ezmsg/sigproc/align.py — AlignAlongAxisProcessor + AlignAlongAxis unit. Extracted all time-alignment/buffering logic (seek-based overlap detection,
  gain matching, per-input buffer management) from the old merge.py. Returns paired aligned chunks (a, b).
• src/ezmsg/sigproc/concat.py — ConcatProcessor + Concat unit + shared helpers. Queue-based FIFO pairing (like Add). Includes:
  • _build_merged_coordinate_axis() — struct-aware axis merging (Req 3)
  • _validate_shared_axes() — assert identical shared axis .data (Req 2)
  • _build_cached_axes() — pre-construct non-alignment axes once (Req 4)
  • New-axis validation: all non-concat dims must match (Req 1)

Rewritten:

• src/ezmsg/sigproc/merge.py — Merge is now an ez.Collection composing AlignAlongAxis → Concat. MergeProcessor is a convenience wrapper preserving the old
  proc(msg_a) / proc.push_b(msg_b) interface.

Tests:

• tests/unit/test_align.py — 8 tests covering aligned pairs, stagger, float precision, gain mismatch, shape changes
• tests/unit/test_concat.py — 22 tests covering helpers (struct merging, validation), ConcatProcessor (basic, relabel, new-axis, assert-identical,
  struct-aware, caching)
• tests/unit/test_merge.py — All 13 existing tests pass (one minor fix: proc.state.merged_concat_axis → proc.concat_state.merged_concat_axis)