docs: detail v0.3.0 roadmap

[Opperiesen]

What changed

• add a complete v0.3.0 roadmap document;
• decompose the milestone into epic Epic: Deliver Lanweave v0.3.0 adapter architecture and optional cloud backend #56 and issues Freeze the v0.3 adapter and capability contracts #57-Prepare v0.3.0 migration documentation and release gates #64;
• define the local-classic compatibility boundary and the opt-in Site Manager read-only cloud scope;
• document capability, profile, plan, MCP, security, evidence and release-gate decisions;
• align the general roadmap and release verification page with v0.3.0.

Why

The previous v0.3.0 scope was too compact to drive implementation safely. This PR makes the adapter architecture, cloud limits and release evidence independently actionable without promising undocumented UniFi endpoints or cloud mutations.

Validation

• uv run pytest -q — 60 passed, 6 skipped;
• uv run ruff check .;
• uv run ruff format --check .;
• git diff --check;
• v0.3.0 milestone checked on GitHub with all child issues assigned.

Closes #56.