Add install-nixie shared action

[lodyai]

Summary

• add a reusable install-nixie composite action with configurable pins for Nixie CLI, Merman CLI, and Python
• prefer locked cargo binstall packages for Merman and fall back to an exact locked cargo install source build when cargo-binstall is unavailable
• export the uv tool binary directory through GITHUB_PATH so later workflow steps can invoke Nixie
• document published and repository-local usage, failure behaviour, version overrides, and the maintainer test contract
• cover both successful Merman paths, missing prerequisites, all installer failures, PATH-export ordering, concrete overrides, and bounded shell-safe version properties
• ignore the pytest-cov .coverage database so validation and stop hooks do not dirty the working tree

Review walkthrough

• Start with .github/actions/install-nixie/action.yml for the unchanged public inputs, installer fallback, and PATH hand-off.
• Review .github/actions/install-nixie/tests/test_action.py for success, prerequisite failures, installer failures, PATH-export ordering, and property-based version coverage.
• Finish with the action README and changelog, the Install Nixie sections in docs/users-guide.md and docs/developers-guide.md, and the root catalogue link.

Validation

• pytest .github/actions/install-nixie/tests/test_action.py: unavailable because pytest is not on the shell PATH.
• uv run pytest .github/actions/install-nixie/tests/test_action.py: passed, 11 tests.
• make check-fmt: passed, 218 files checked.
• make typecheck: passed.
• make lint: passed, including Ruff, action-validator, Whitaker, and spelling checks.
• make test: passed, 1,068 tests passed and 14 skipped; auxiliary typos tests passed 3 tests with 95.45% coverage.
• make markdownlint: passed, 73 Markdown files with zero errors.
• make nixie: passed, all Mermaid diagrams valid.
• git diff --check: passed.

References

• Wildside Nixie and Merman toolchain PR
• Lody session

Summary by Sourcery

Introduce a reusable Install Nixie action for reliably provisioning the Mermaid validation toolchain in GitHub Actions.

New Features:

• Add a reusable composite action for installing pinned Nixie and Merman CLI toolchain releases with configurable Nixie, Merman, and Python versions.

Enhancements:

• Install Merman through locked binary packages when available and fall back to a locked source build otherwise.
• Make the installed Nixie executable available to subsequent workflow steps through GITHUB_PATH.

Documentation:

• Document published and repository-local usage, version overrides, prerequisites, failure behavior, and maintenance expectations.
• Add the action to the repository's GitHub Actions catalogue.

Tests:

• Add behavioral and property-based coverage for installation paths, overrides, prerequisites, failures, and PATH export ordering.

Chores:

• Ignore the pytest-cov coverage database to keep validation hooks from dirtying the working tree.