Tighten 2-isogeny descent local solubility checks (improve rank upper bounds)

[asiryan]

Motivation

• The lightweight rank-upper bound (2-isogeny descent) produced very coarse bounds in nontrivial cases due to heuristic/skipped local tests and a weak 2-adic sieve.
• Improve correctness of local solubility screening for the quartic coverings that appear in the 2-isogeny Selmer enumeration so the computed upper bounds are tighter.
• Avoid unsafe casting of large prime factors when building the list of local primes to test.
• This change focuses on the local-solubility stage used by RankBounds/TwoIsogenySelmerDimension (partial improvement toward a full descent implementation).

Description

• File modified: sources/EllipticCurveQ.Analysis.cs.
• Replace the previous LINQ cast of prime factors into int with an explicit loop that filters out p<2 and returns null if any prime exceeds int.MaxValue, avoiding unsafe casts.
• Remove the previous heuristic skip for very large primes and add a deterministic per-prime routine HasSolutionModuloPrime to check solvability modulo p (and handle singular roots via a lift to p^2).
• Handle the special case when p | d in the quartic by reducing the equation and checking the correct residue classes; add IsSquareInQp to detect trivial solvability via a Q_p square test for b/d.
• Increase 2-adic modulus used for the 2-adic local check from 2^6 (64) to 2^8 (256) to reduce missed 2-adic obstructions; add helpers ModToLong / reuse ModToInt for safe modular reductions.

Testing

• No automated tests were executed as part of this change.
• Suggested validations (manual/automated to run after this PR): run dotnet build, execute the examples program, and compare RankBounds results on known curves against trusted data (e.g. LMFDB) to verify improved upper bounds.

Notes: this PR tightens local solubility checks used in the 2-isogeny descent and should reduce false-positive Selmer classes; it is not a complete implementation of all descent methods (full multi-descent / global 2-descent is not yet implemented).

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