Scaffold TestMining.Platform solution structure and shared project conventions

[kalen-releasedgroup]

Summary

• scaffold the initial TestMining.Platform.* vertical slice under src/ and tests/ without repurposing retained Symphony.* assets
• add shared repository build conventions for nullable reference types, deterministic builds, and .NET analyzers
• lock the intended platform project reference boundaries and solution membership with platform-focused smoke/conformance tests

Concern

Primary product: semantic test mining platform

Requirement Links

• Requirements section 6
• Requirements section 17
• Requirements section 22.1
• Requirements section 22.2
• Requirements section 23.1 item 6

What Changed

• added empty scaffold projects for host, core, application, recording, analysis, generation, replay, healing, persistence, artefacts, contracts, and recorder
• added matching test projects and assembly-marker smoke tests to keep the scaffold wired into the solution
• updated Symphony.slnx to include the new platform projects while keeping retained Symphony tooling separate
• added solution conformance coverage that asserts expected project membership, reference graph, and inherited repository conventions
• introduced repository-wide defaults for nullable, deterministic builds, and analyzers via Directory.Build.props

Validation

• dotnet build Symphony.slnx
• dotnet test Symphony.slnx --no-build

Sensitive Data / Outbound Calls

• no new sensitive data fields
• no new outbound network calls in product code

Closes #4