RFC: Decompose the four grade-D complexity hotspots

[mmcky]

Problem

Radon rates exactly four functions at cyclomatic-complexity grade D — everything else in the package is C or better, so the complexity is concentrated and tractable:

Function	Location	CC
_compute_fixed_point_ig	_compute_fp.py:159	25
DiscreteDP.__init__	markov/ddp.py:299	24
brent_max	optimize/scalar_maximization.py:5	24
polym_lcp_solver	game_theory/howson_lcp.py:36	23

DiscreteDP.__init__ alone dispatches dense/sparse × full/state-action-pairs input forms inline; _compute_fixed_point_ig interleaves algorithm and bookkeeping. High-CC plus hand-verified numerics is where regressions hide.

Proposed change

One PR per function; behavior-preserving extraction only:

• DiscreteDP.__init__ → extract _parse_full_form(...) / _parse_sa_form(...) validators returning a normalized bundle.
• _compute_fixed_point_ig → separate the iteration loop from error/logging bookkeeping.
• brent_max is Numba-jitted — some branching is inherent to Brent; treat as best-effort.
• Guard with before/after output equality on existing tests (coverage in these regions is strong).

Related context: #480 (repeated logic in several modules).

Acceptance criteria

• Radon grade ≤ C for the three non-jitted functions; no public API change
• Suite green with unchanged numerical outputs

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From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).