feat(init): guide and certify corpus federation

[tcballard]

Summary

• add opt-in decided init --parent-corpus guidance for fresh and existing repositories, with or without a profile, without creating a manifest, fetching, or touching parent bytes
• preserve an existing non-default repository key when guidance uses the default key while retaining explicit-key conflict behavior
• adapt only the composed local layer back to writable physical-path conventions for rename and local OKF consumers; inherited records remain read-only
• make parallel CLI scratch fixtures collision-free across coarse-clock platforms, including the macOS native smoke runner
• add a 14-case black-box certification harness covering verification, containment, collisions, qualification, overrides, inherited routing/enforcement, exports, no-manifest parity, and the full mutation boundary
• extend the parent-inheritance requirement's existing profile-guidance clause without changing its Proposed status
• keep the corpus-federation roadmap Planned until this stack is reviewed and merged

Stack

• depends on feat(export): compose inherited corpus projections #460
• final implementation/certification PR in the federation stack

Closes #271
Closes #267

Validation

• decided CLI: 15/15
• black-box federation certification: 14/14
• full asdecided-core suite: all unit and integration tests pass; one intentional differential test remains ignored
• cargo clippy -p asdecided-core -p decided -p decided-mcp --all-targets -- -D warnings
• complete parent-tree byte snapshots before and after every mutation command
• exact no-manifest resolve and export parity
• git diff --check

All listed checks pass.