Add IR property tests (#322)

[lodyai]

Summary

This branch adds property-based coverage for the invariants introduced around
PR #315: IR cycle detection now has generated DAG, cycle, missing-dependency,
order-only exclusion, and insertion-order determinism checks; Ninja build-line
formatting now checks dependency separator ordering; and command interpolation
now checks placeholder substitution around backtick-delimited regions.

Closes #322.

It also centralizes the manifest placeholder tokens used by rendering and
interpolation so the tests and implementation share one source of truth.

Review walkthrough

• Start with src/ir/cycle_property_tests.rs for the generated graph invariants.
• Review src/ninja_gen_property_tests.rs for the Ninja separator ordering checks.
• Review src/ir/cmd_interpolate_property_tests.rs for backtick and placeholder interpolation properties.
• Check src/ir/cmd_interpolate.rs and src/manifest/render.rs for the shared placeholder constants.

Validation

• cargo test --all-targets --all-features property_tests: passed, 11 property tests.
• make check-fmt: passed.
• make lint: passed.
• make test: passed.
• coderabbit review --agent: run after each review milestone; all substantive concerns were addressed before this PR was opened.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-322-property-tests-for-cycle-detection-ninja-separator-ordering-and-command-interpolation

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