manifest: define and validate module dependency stability closure

[codeforester]

Goal

Define and validate the stability contract for module dependency closure.

Background

The current API manifest marks gh as stability: stable and since: 2.0.0, but its current implementation declares process as a dependency. The process module is explicitly stability: preview, since: unreleased, and is not present in the immutable v2.0.0 tree. scripts/api-manifest check validates dependency existence and cycles, while release-check verifies only stable module paths and symbols; neither checks the stability of a stable module's dependency closure. This leaves it ambiguous whether stable gh is consumable as a stable API, whether its dependency must be promoted with it, or whether the module itself should remain preview.

Scope

• Specify whether stable modules may depend on preview/unreleased modules.
• Extend manifest validation and release-check to enforce the chosen policy transitively and report the offending dependency path.
• Align base_api_manifest.yaml, module READMEs, API reference, and v2 contract documentation with that policy.
• Add a regression fixture for a stable-to-preview dependency and a valid stable closure.

Acceptance Criteria

• The manifest has one unambiguous rule for dependency stability and release availability.
• CI fails before merge when a stable module violates that rule, or the manifest explicitly models a reviewed exception with coverage and release behavior.
• A release-check cannot report a stable API as valid while its required dependency is absent from the referenced release tree.
• Current gh/process metadata and documentation are made consistent with the selected policy.

Validation

• ./tests/validate.sh
• Focused tests/api-manifest.bats coverage for stable/preview dependency closure and release-tree checks.

Non-Goals

• Promoting process to a stable release as part of this issue.
• Changing the public GitHub helper signatures.

Project Fields

• Status: Backlog
• Priority: P1
• Area: Manifest
• Initiative: Adoption Polish
• Size: S
• Milestone: v2.1.0

Agent Assignment

• Assignee: codeforester