Reduce redundant host work in the xFit LM solver

[tpn]

Summary

The normal-equation path copied five arrays even when every row was valid,
and filtered trial-evaluation budgets after analytic Jacobians that consume no
residual evaluations. Skip those operations to reduce repeated host work in
xFit's shared solver. Preserve fallback filtering and finite-difference budget
checks, with a regression for exhausting the budget just before a trial in both
implicit and explicitly requested finite-difference modes.

Validation

• make ci-lint — passed, including mypy, Ruff, pre-commit and lock validation.
• make package-check — source distribution built; strict package checks passed.
• pytest -q tests/xfit/test_solver.py tests/xfit/test_numba_mlir.py tests/xfit/test_numba_mlir_commands.py — 67 passed, no skips, with CUDA 13 and free-threaded Python.

With free-threaded Python 3.14 and Numba-CUDA-MLIR 0.5.4, one concurrent
warmup attempt failed in dependency type dispatch. Performance validation
covers successful warmed fits; the effect of this change on concurrent-startup
reliability remains undetermined.

Repository hygiene

• No credentials, private paths, local datasets, notebook outputs, or generated run artifacts were added.
• Large binary artifacts are excluded or intentionally tracked through an approved storage plan.
• Security-sensitive information is not included.
• User-visible behavior changes are documented.
• New dependencies and their licenses are documented.

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