Stage 3.9: ICA redesign - dqdv/dvdq on one pure core, IcaOptions, specced output frame

[jepegit]

Goal

Execute the ICA redesign: one pure dqdv() core plus a new dvdq(), an IcaOptions
dataclass replacing the ~20 tunneled kwargs, and a specced long-format output frame.

Why

utils/ica.py (1 124 lines) has four entry points that accept the same twenty options
in three different ways, and returns four different shapes (numpy tuples, long frames,
wide MultiIndex pairs). More importantly the output frame schema is a data contract:
users consume the returned columns directly. Changing it in 2.1 would be a break in a
minor release, so it has to happen at 2.0.

Plan: architecture-plan/cellpy2-ica-redesign-plan.md.

Scope

• One pure core over cellpycore.curves; scipy stays app-side.
• IcaOptions dataclass — the only way to pass converter parameters.
• dqdv() and new dvdq() with symmetric signatures.
• Specced long-format output frame with native column names (a CurveCols-style
  spec, documented in the data-format specifications).
• Old entry points (dqdv_cycle, dqdv_cycles, dqdv_np, split=True) kept as
  warn_once shims registered in DEPRECATIONS.md, removal at 2.1.

Acceptance

• The 19 existing tests/test_ica.py characterization tests pass through the shims.
• New tests assert the output frame against its spec.
• dvdq() on a golden cell matches an independently computed reference.
• No hidden converter state between calls (determinism test).