Merge GetNative refactor series (#61-#65)

[General-Fault]

Summary

Merge the GetNative refactor series into main:

• Refactor Media types: scoped_refptr GetNative* pattern #61 Media types use scoped_refptr-based GetNative*
• Refactor RtcDataChannel: scoped_refptr GetNative* pattern #62 RtcDataChannel uses scoped_refptr-based GetNative*
• Refactor RtcIceTransport: scoped_refptr GetNative* pattern #63 RtcIceTransport uses scoped_refptr-based GetNative*
• Refactor RtcPeerConnection and stub transport/transceiver types: scoped_refptr GetNative* pattern #64 Peer/stub types use scoped_refptr-based GetNative*
• Document GetNative* refactor: create ADR and update documentation #65 Documents the new pattern with ADR-0004 and shared guidance updates

Fixes #61
Fixes #62
Fixes #63
Fixes #64
Fixes #65

Change Area (required)

• API-only managed change (WebRtcNet.Api / managed-only behavior)
• Interop / marshaling / native-boundary change
• Docs/infra only (no behavior change)

Required Evidence (for behavioral/API changes)

N/A - no external WebRTC or Media Capture behavior change.

Test Evidence (required)

Tests added/updated

• No new unit tests were added for these slices.

Commands run + results

• dotnet test WebRtcNet.Api.UnitTests\WebRtcNet.Api.UnitTests.csproj — passed (110 tests)
• msbuild WebRtcInterop\WebRtcInterop.Framework.vcxproj -p:Configuration=Debug -p:Platform=x64 — succeeded
• msbuild WebRtcInterop.UnitTests\WebRtcInterop.UnitTests.vcxproj -p:Configuration=Debug -p:Platform=x64 — succeeded
• WebRtcInterop.UnitTests\bin\x64\Debug\v4.8\WebRtcInterop.UnitTests.exe — passed (58 tests)

Change-area test gate confirmation

• API-only managed change: ran managed NUnit tests
• Interop/native-boundary change: ran interop unit tests (when environment available)
• Required environment unavailable (explain below)

If environment unavailable (required when checked above)

• Missing environment/dependency: N/A
• What was run instead: N/A
• Residual risk: None.

Documentation

• I updated documentation for externally visible behavior changes.
• No external docs impact.

Spec/Crosswalk Review (required for W3C-aligned behavior changes)

• Reviewed docs\\standards\\crosswalk\\webrtcnet-api-to-spec.md
• Reviewed docs\\standards\\specs\\index\\spec-map.md
• Refreshed/checked local spec snapshots (.\\scripts\\update-spec-docs.ps1) when needed