fix: snapshot spline public array state

[0xC000005]

Summary

• harden ChebyshevSpline.Domain, NNodes, and Knots so public getters return snapshots
• keep internal hot paths on backing fields/storage accessors instead of cloning through public getters
• add regression coverage for public mutation, internal storage accessors, and null setter branches

Related to #73. This is a staged partial fix and does not close the issue.

Verification

• RED: SplinePublicStateOwnershipTests failed 2/2 before the production change
• dotnet test tests/ChebyshevSharp.Tests/ChebyshevSharp.Tests.csproj --configuration Release --filter FullyQualifiedName~SplinePublicStateOwnershipTests --verbosity minimal: 4/4 passed
• dotnet test tests/ChebyshevSharp.Tests/ChebyshevSharp.Tests.csproj --configuration Release --filter "FullyQualifiedNameSpline|FullyQualifiedNameSpecialPoints|FullyQualifiedNameAutoKnots|FullyQualifiedNameExtrudeSlice|FullyQualifiedNameCalculus|FullyQualifiedNameBinaryFormat" --verbosity minimal: 403/403 passed
• dotnet test tests/ChebyshevSharp.Tests/ChebyshevSharp.Tests.csproj --configuration Release --verbosity minimal: 1520/1520 passed
• dotnet restore: passed for fresh worktree benchmark/example assets
• dotnet build --configuration Release --no-restore --verbosity minimal: passed, 0 warnings/errors
• dotnet format ChebyshevSharp.slnx --verify-no-changes --verbosity minimal: passed
• dotnet pack src/ChebyshevSharp --configuration Release --no-build --output /tmp/chebsharp-cycle12-spline-pack: passed
• git diff --check: passed

Reference/Design Notes

• Microsoft CA1819 notes array-returning properties are not write-protected and require copies to be tamper-proof.
• Microsoft CA2227 reinforces read-only collection/property contracts when mutation is not intended.
• The fix follows the PR Snapshot Approximation public array state #77 pattern: public snapshots plus internal live storage, avoiding hidden clone allocations in evaluation/arithmetic paths.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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