Adopt MoreLINQ; remove duplicated collection extensions

[ChrisonSimtian]

Tier 2 — Consolidate. src/Fallout.Utilities/Collections/ (~900 LOC, 23 files) contains many LINQ-like helpers that duplicate MoreLINQ: Enumerable.Distinct.cs (DistinctBy), Enumerable.Random.cs (Random/Shuffle), Enumerable.TakeUntil.cs, Enumerable.ForEach.cs.

Action

Add MoreLINQ; delete the duplicated helpers and redirect call sites.

Keep (genuinely Fallout-specific — do NOT delete)

• Enumerable.TopologicalSort.cs (cycle-detecting sort used for target ordering)
• LookupTable.cs (mutable ILookup — no BCL/MoreLINQ equivalent)
• Enumerable.SingleOrDefaultOrError.cs / Enumerable.ToDictionary.cs (custom diagnostic messages)

⚠️ Breaking change

These are public extension methods. Removal breaks consumers → breaking-change, experimental, batched to yearly major, CHANGELOG migration note. Where feasible, keep [Obsolete] forwarders.

Acceptance

• MoreLINQ added to Directory.Packages.props
• Duplicated Distinct/Random/TakeUntil/ForEach helpers removed or obsoleted
• Fallout-specific helpers retained
• CHANGELOG entry + migration note

From dependency-consolidation audit, 2026-06-02.

[ChrisonSimtian]

Insights from a discarded prototype (old paths; map to onion structure after #359).

• Package: MoreLinq 4.4.0 (central).
• Only 2 genuine duplicates: DistinctBy and Shuffle (was Randomize). Replace via [Obsolete] forwarders.
• KEEP these — not real duplicates:
  • TakeUntil/SkipUntil: ours is EXCLUSIVE, MoreLINQ's is INCLUSIVE. Swapping silently changes ArgumentParser + CompletionUtility.
  • ForEach/ForEachLazy: MoreLINQ has no Action<T,int> index overload and no ForEachLazy; ~70 callers.
  • single-pick Random<T>(): no MoreLINQ/BCL equivalent.
• Gotcha: net10 BCL now ships DistinctBy/Shuffle, which collide with MoreLINQ's extensions. Call MoreLinq.MoreEnumerable.* explicitly so both netstandard2.0 and net10.0 resolve.