Extract manifest contracts from base_setup behind a neutral package boundary

[codeforester]

Goal

Move Base manifest contracts behind a neutral Python package boundary so unrelated subsystems no longer depend on setup internals.

Background

Manifest models, loading, schema constants, validators, and compatibility imports currently live under base_setup. Roughly thirty non-test production files outside that package import it. This makes base_setup the implicit domain core and obscures the intended dependency direction.

Closed issues #288, #1420, #1442, and #1461 split manifest internals, while #1576 extracted devcontainer and devenv consumers. None established a neutral manifest package or mechanically enforced the package dependency graph.

Scope

• Define a focused package such as base_manifest owning the model, loader, schema constants, validators, and public read API.
• Preserve documented compatibility imports during migration.
• Move consumers incrementally without duplicating schema definitions.
• Document the intended Python package dependency DAG.
• Add import-boundary or structure tests that reject wrong-direction dependencies.
• Identify any setup-specific manifest reconciliation that should remain in base_setup.

Acceptance Criteria

• New code can consume manifest contracts without importing base_setup.
• Manifest parsing behavior, errors, and dataclass output remain compatible.
• Compatibility facades are explicit and tested.
• The allowed dependency direction is documented and mechanically checked.
• At least one major non-setup consumer migrates to the neutral package.

Validation

• Full manifest parser and compatibility tests.
• Package structure/import-boundary tests.
• Focused tests for migrated consumers.
• Pylint on touched production Python.
• git diff --check.

Non-Goals

• Do not create a vague catch-all base_core package.
• Do not redesign the manifest schema.
• Do not extract a separately published package in the first slice.

Project Fields

• Status: Backlog
• Priority: P2
• Size: L
• Area: Python
• Initiative: Contract Hardening

Agent Assignment

Human first. Establish the dependency boundary and compatibility plan before moving consumers.