Activate the Aqua distribution channel

[konojunya]

Summary

• pin the owner Aqua registry to immutable commit 42702cda91a4156901b9a601bd143c43dcf05766
• publish a narrow policy, checksum-lock workflow, install commands, and update procedure
• mark Aqua available for all four tier-1 release targets in the distribution contract
• validate the registry revision, target mapping, and five checksum entries in CI

The registry consumes the existing v0.3.0 GitHub Release archives without replacing or repacking any asset. Cargo and self-update remain planned.

Verification

• Aqua 2.56.1 remote github_content install tests for darwin/amd64, darwin/arm64, linux/amd64, and linux/arm64
• isolated macOS arm64 user flow: aqua policy allow, aqua update-checksum, aqua install, stack --version, init, check, and render
• stable aqua update resolution remains pinned to v0.3.0
• node scripts/validate-distribution-contract.mjs
• node --test scripts/distribution-contract.test.mjs scripts/aqua-registry.test.mjs
• cargo fmt --check
• cargo test --locked
• git diff --check