feat(cuda): add LLamaSharp.Backend.Cuda13 packages

[modelpath-dev]

Why

Part 3 of the CUDA 13 split for #1360. Ships the Backend.Cuda13 nuspecs, Runtime.targets copy rules for cu13.0.0, and package docs. Hold this until the NuGet ownership issue Martin mentioned is resolved and cuda13 binaries exist from part 1.

Depends on #1440.

Scope

• LLama/runtimes/build/LLamaSharp.Backend.Cuda13*.nuspec
• LLama/LLamaSharp.Runtime.targets copy-to-output links for cu13.0.0 → cuda13
• README.md, docs/QuickStart.md package mentions

Out of scope: compile workflow and NativeLibraryWithCuda (parts 1 and 2).

Tradeoffs

Runtime.targets sits with the NuGet packaging change, not the loader, because it is copy-to-output packaging for deps, not Prepare() probe logic.

Blast Radius

Adds new packages and optional copy rules gated on Exists(...). Existing Cuda12 packages and paths are untouched. Publishing still waits on package ownership.

Verification

• NuGet-only files vs the loader tip: targets, three nuspecs, README, QuickStart.
• Full tip patch-id matches the pre-split combined change that used to live on this stack.
• No local pack run (needs binaries from Compile). Review against the Cuda12 nuspec/targets pattern.

[martindevans]

Our nuget key issue has finally been sorted out, so I think I can merge this now 🥳

Could you rebase it onto master before I give it a final review please.

[modelpath-dev]

@martindevans rebased onto master. Since #1439 and #1440 already landed, the PR is down to the one packaging and docs commit, and I checked that every cu13.0.0 file the new nuspecs and targets reference is in the 815a2a5915f22c_withcuda13 binaries release.