docs: close v1.6.0 release roadmap

[vntrevx]

Outcome

Close the completed v1.6.0 Full Native Strategy release in the public documentation and authoritative roadmap state.

Changes

• point README release/install status and checksums to v1.6.0
• record same-commit CI, candidate build, RC publication, stable promotion, 3OS installer audits, and byte-identical assets
• mark M22-04 completed and clear the active roadmap task
• retain the independent Spot/Futures certificate and external 2022-report claim boundaries

Validation

• uv run pytest -q tests/test_release_contract.py (29 passed)
• uv run ruff check .
• git diff --check
• v1.6.0 is the GitHub latest release at commit 87bc580c5fee1700f36fc78475c03ce9a94485cc

Documentation/roadmap only; no runtime, release asset, or evidence mutation.

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