[v1.0] Formalize typed extension plugin protocols and compatibility negotiation

[codeforester]

Goal

Make third-party extensions safe to build, version, test, and evolve against a documented plugin SDK.

Background

Entry-point extensions and lifecycle hooks matter for adoption, but without typed protocols, capability metadata, and negotiation, plugins must rely on internals.

Scope

Define public protocols for each extension group; publish API version and capabilities; add compatibility negotiation and actionable rejection errors; provide a fixture plugin/test harness and collision/disable behavior; document stability and deprecation.

Acceptance Criteria

• Every extension group has a typed contract and fixture.
• Incompatible versions are rejected before execution.
• Capability discovery is deterministic.
• Lazy loading, allowlists, and collision handling remain intact.

Validation

Build/install a fixture plugin wheel; test compatible, incompatible, disabled, and colliding plugins; type-check public protocols and run the full suite.

Non-Goals

Do not expose private implementation modules as a substitute for a supported SDK.

Project Fields

• Status: Backlog
• Priority: P1
• Area: Python
• Initiative: v1.0 Readiness
• Size: M

Ownership

• Assignee: @codeforester
• Related work: app.py concentrates ~3.2k lines / 4 distinct concerns in one file — split before locking the v1.0 API #180 (app.py decomposition) should land before freezing additional public extension surfaces.