feat: add checkpointed resumable execution

[szmyty]

Summary

• persist corruption-detectable renderflow.checkpoints/v1 node checkpoints immediately after atomic artifact storage and successful validation
• assess checkpoint compatibility against source identity, transform/configuration and implementation versions, provider/toolchain evidence, and verified output bytes
• resume deterministic/cacheable DAG nodes independently while forcing missing, corrupt, changed, or invalidated work to recompute
• expose the versioned RenderflowProvider Rust contract for capability inspection, planning, running, assessment, resume, and explicit invalidation
• enrich renderflow.progress/v1 events with run/step/artifact IDs, states, and diagnostics, and propagate cancellation to bounded DAG-wave boundaries
• add renderflow build --resume, JSON schemas, and provider/recovery documentation

Verification

• cargo check --workspace --all-targets --locked
• changed Rust files pass rustfmt --check
• JSON schemas and MkDocs YAML parse successfully
• smoke build records a validated checkpoint; an unchanged renderflow build --resume reports state: reused, cache: hit, and preserved validation status

The broad test/Clippy/CI pass is intentionally deferred under the current sprint mode; all-target compilation includes the new checkpoint fixtures.

Closes #358