Scaffold TestMining.Platform.* solution structure and shared project conventions

[kalen-releasedgroup]

Summary

Introduce the new TestMining.Platform.* vertical slice without mutating retained Symphony.* tooling assets.

Scope

• Create the initial platform project layout described in docs/technical-specification.md.
• Enable shared repository conventions for nullable, analyzers, and deterministic builds where practical.
• Establish reference boundaries between host, core, application, recording, analysis, generation, replay, healing, persistence, artefacts, contracts, and recorder projects.

Requirement Links

• Sections 6, 17, 22.1, 22.2
• Section 23.1 item 6

Dependencies

• None

Exit Criteria

• New platform projects exist and build as an empty skeleton.
• Project references reflect the intended architecture boundaries.
• No retained Symphony.* assets are repurposed.

[kalen-releasedgroup]

Working on this in branch symphony/4. I’m checking linked PR context and repository state before making the code change.