feat(storage): plan retained manifest imports

[fishmingyu]

Summary

Add a pure, backend-neutral planner that converts bounded RepoManifest v1.1 input into immutable retained-import intent. This layer validates and selects portable artifacts, but deliberately does not read artifacts, write CAS objects, mutate the catalog, or publish refs.

Changes

• Add immutable source, view, selection, and import-plan value types plus mapping and byte-oriented planning entry points.
• Parse manifest JSON under strict UTF-8, ambiguity, size, depth, node, key, atom, integer, and selection bounds.
• Admit only current BM25 schema 8 and vector schema 6 artifacts with complete generation and builder-profile identity; required views fail closed while optional views record explicit skip decisions.
• Treat source fingerprint v2 as eligible input for later authentication and keep v1 plans inert; reject diagnostic and authority claims embedded anywhere in the manifest.
• Charge remaining encoded-byte budgets before reading caller-owned mapping children, and expose the planner through the lazy compiler API.
• Record the pure planning boundary while keeping retained reads, CAS upload, catalog mutation, and ref publication explicitly outstanding.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring
• Performance improvement
• Tests

Testing

• Tests pass locally

• Added new tests for the changes

• pytest -q test/compiler/test_manifest_storage.py --tb=short: 256 passed.

• pytest -q test/compiler test/storage/test_models.py --tb=short: 640 passed.

• Local unit tier on the equivalent restacked patch: 5815 passed, 71 skipped, 191 deselected.

• pre-commit run --from-ref origin/main --to-ref HEAD: passed.

• python -m mkdocs build --strict: passed.

• git diff --check origin/main...HEAD: passed.

Checklist

• My code follows the project style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published

#606 is merged and its exact-head and post-merge gates are green. This PR is restacked directly on the resulting main. The planner is intentionally authority-free and does not enable a production retained-manifest importer.