feat: add typed artifact plugin SDK v2

[szmyty]

Summary

• introduce the explicit renderflow.plugin/v2alpha1 artifact-native transform contract
• support typed single artifacts and ordered artifact collections without UTF-8 conversion
• provide scoped immutable artifact access, bounded process execution, cancellation, progress, execution identity, structured diagnostics, metrics, and provenance
• enforce semantic transform versions, declared capabilities/formats, deterministic cache eligibility, loss profiles, and explicit duplicate/replacement policies
• wire v2 single and collection plugins into the canonical DAG while preserving the v1 text-plugin adapter
• add read-only lifecycle observers that cannot mutate artifacts, plans, validation, approval, or release state
• add a descriptor JSON Schema, compatibility policy, migration documentation, and text/binary/collection/external-tool examples

Sprint validation

Per the current speed-run workflow, broad tests and CI were intentionally deferred.

• cargo check --workspace --all-targets --locked
• changed Rust files formatted with rustfmt
• staged diff passes git diff --check
• credential-pattern scan found no committed secrets

The focused v2 contract fixtures compile as part of --all-targets, but were not executed. Broader conformance and regression work remains coordinated through #366 and the sprint-end test/polish follow-up.

Closes #357

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