Add artifact validation, fidelity gates, and a capability conformance matrix

[szmyty]

Outcome

Require Renderflow to prove that an output is structurally usable and policy-compliant before it is reported as a successful artifact.

Format support should mean a tested capability, not merely an enum variant or an external command that happened to exit zero.

Scope

Validation model

Define structured validation outcomes that can represent:

• valid
• valid with warnings
• invalid
• validator unavailable
• validation skipped by explicit policy

Validation evidence should identify the validator implementation/version and preserve raw diagnostics safely where useful.

Fidelity/loss policy

Make transform loss characteristics enforceable:

• lossless
• partial loss
• lossy
• path-dependent

Allow specs/profiles to set minimum fidelity requirements or reject particular loss classes.

Capability conformance

Generate a machine-readable matrix for each declared format/capability/transform covering:

• implemented executor
• required provider/tool
• fixture coverage
• validator
• supported platforms
• deterministic status
• loss profile
• current support status

Do not advertise a capability as production-supported when its required fixture or validator evidence is absent.

Initial validation adapters

Use narrow, replaceable validators appropriate to each family. Evaluate existing/native parsers first and wrap external validators through #356/#359 where required. Examples may include structural probes for media, document/PDF, EPUB, images, archives, and HTML.

Do not make a specific third-party validator canonical without evaluating licensing, portability, maintenance, and failure semantics.

Acceptance criteria

• Every terminal artifact has an explicit validation state in execution evidence.
• A transform exiting successfully cannot produce a complete artifact when required validation fails.
• Validators are capability/tool providers with versions and structured diagnostics.
• Specs/profiles can require validation and reject configured loss classes.
• Validation failure can be fatal or branch-local according to explicit execution policy.
• The generated capability matrix distinguishes implemented, experimental, unavailable, and planned behavior.
• Documentation and product-site capability claims can be derived from the conformance matrix.
• Representative fixtures cover document, image, audio/video, archive, and structured-data validation.
• Corrupt/truncated outputs are caught by tests even when the producing fake command exits zero.
• Validation evidence participates in checkpoint/resume compatibility.
• Formatting, Clippy, tests, docs, and CI pass.

Dependencies

Depends on #352 and #359; execution evidence should integrate with #355.