[Artifact Forest 1/9] Define cycle-safe exhaustive derivative planning

[szmyty]

Parent: #421

Outcome

Add a canonical planning policy for "build every eligible derivative" without recursively walking cycles or producing duplicate/source-equivalent artifacts.

Problem

Renderflow's graph intentionally supports reversible and competing edges. A naive exhaustive traversal over edges such as fountain -> fdx -> fountain can loop, duplicate work, and create lower-fidelity derivatives from already-derived artifacts.

Model

Introduce a derivative-forest planning mode that is target-oriented.

For a source artifact and policy:

1. enumerate eligible target artifact identities/capabilities;
2. exclude source-format identity by default;
3. compute one selected simple path per target;
4. forbid repeated format nodes inside a selected path;
5. deduplicate equivalent target identities;
6. prefer higher-fidelity/direct source paths over paths starting from another derivative;
7. preserve unavailable/unsupported targets as diagnostics rather than outputs.

Example:

source: fountain
targets: fdx, fadein, osf, json, pdf

fountain ─→ fdx
         ├→ fadein
         ├→ osf
         ├→ json
         └→ pdf

Deliverables

• Versioned derivative-forest plan/evidence schema.
• Explicit source-exclusion and target-deduplication rules.
• Simple-path/cycle policy.
• Target identity beyond raw extension where variants/profiles matter.
• Loss/fidelity budget and path-selection evidence.
• Provider-availability filtering.
• CLI/SDK dry-run inspection for the forest.
• Deterministic fixtures including reversible A ↔ B transforms and competing A → C paths.

Acceptance criteria

• Reversible graph edges cannot cause recursive execution or duplicate target artifacts.
• The source format is not regenerated unless explicitly requested.
• Each target identity is materialized at most once per source/profile.
• Selected paths contain no repeated format node.
• Higher-fidelity source-derived paths win according to documented policy.
• Unsupported/unavailable targets remain explicit.
• Plan identity includes source, policy, targets, provider/toolchain identity, and relevant options.
• Existing targeted graph execution remains compatible.
• Tests cover cycles, aliases, competing paths, unavailable providers, lossy paths, and explicit source-format requests.

Non-goals

No new screenplay, subtitle, or AI provider is implemented here.