P2.7 Dissipative forces: backend test scaffold (migration deferred to Pspecial)

[jobovy]

Part of the P2.x series (→ #892). Dissipative / velocity-dependent forces (f=f(x,v)).

Outcome (conservative, by design): zero compute methods migrated — every force path in ChandrasekharDynamicalFrictionForce and FDMDynamicalFrictionForce irreducibly uses scipy.special.erf/sici + a scipy.interpolate sigmar spline + a mutable input-hash cache + data-dependent branching, all out of scope until the Pspecial backend router. No source files changed → numpy path byte-identical by construction.
Deliverable: tests/test_backend_dissipative.py — 6 passed, 2 strict-xfail. Establishes the v=[vR,vT,vz] calling convention + _cached_force*{...} factoring + below-minr-zero branch, pins the numpy values as a regression guard, and documents (xfail) that jax.grad currently raises TracerArrayConversionError (auto-flips to PASS once Pspecial + a cache/branching refactor land).
Follow-up: after Pspecial provides backend erf/sici and a backend-friendly sigmar, refactor the cache (gate on xp is numpy) + branches (xp.where), then migrate _calc_force + add real parity/grad tests. (This is also "route 2" — the C-STM dissipative Jacobian — on the variational side.)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.92%. Comparing base (141ebfe) to head (e0d6567).

Additional details and impacted files

@@              Coverage Diff               @@
##           feat/backends     #897   +/-   ##
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  Coverage          99.92%   99.92%           
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  Files                228      228           
  Lines              34213    34213           
  Branches             712      723   +11     
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  Hits               34186    34186           
  Misses                27       27           

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[jobovy]

Closing per review: the dissipative-forces test scaffold on its own isn't worth merging. The real DissipativeForce/ChandrasekharDynamicalFriction/FDMDynamicalFriction namespace-swap (P2.7) will be done as a proper migration after Pspecial lands (some of these depend on special functions), and the tests will go in with that work. Superseded by the upcoming post-Pspecial dissipative migration.