Snapshot TT public array properties

[0xC000005]

Summary

• Return snapshots from ChebyshevTT.Domain and ChebyshevTT.NNodes instead of exposing private invariant arrays.
• Add regression tests proving external mutation of those returned arrays cannot corrupt TT metadata or evaluation-domain validation.

Refs #73.

Why This Is A Small Slice

Issue #73 is broader than TT. This PR intentionally hardens only the low-risk TT surface because ChebyshevTT already stores domain and node counts in private backing fields, so snapshot getters do not rewrite hot internal code paths.

Approximation, Spline, and Slider still expose mutable arrays through public auto-properties. They should be handled in follow-up work with a backing-field/snapshot design; a one-line clone-on-get change there would add hidden allocations throughout internal evaluation/build paths.

Verification

• Red focused tests before fix: TtPublicStateOwnershipTests failed 3/3 because public arrays were live.
• dotnet test tests/ChebyshevSharp.Tests/ChebyshevSharp.Tests.csproj --configuration Release --no-restore --filter FullyQualifiedName~TtPublicStateOwnershipTests --verbosity minimal -> 3/3 passed.
• Related TT tests -> 53/53 passed.
• dotnet test --configuration Release --no-restore --verbosity minimal -> 1505/1505 passed.
• dotnet format --verify-no-changes --verbosity minimal -> passed.
• First Release build failed in the fresh worktree because benchmark/example assets were missing; after dotnet restore, dotnet build --configuration Release --no-restore --verbosity minimal passed with 0 warnings/errors.
• dotnet pack src/ChebyshevSharp --configuration Release --no-build --output /tmp/chebsharp-cycle12-tt-pack -> created ChebyshevSharp.0.12.0.nupkg.
• git diff --check -> passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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