refactor(planner): unify canonical execution lifecycle

[szmyty]

Summary

Unifies Renderflow's legacy build path and graph build path behind one canonical planner/executor lifecycle.

Architecture

• adds one application-layer planning module that owns config/spec loading, v1→v2 normalization, target resolution, policy filtering, DAG construction, provider/toolchain resolution, execution, output naming, and diagnostics
• routes renderflow build, --target, --all, graph/inspect commands, SDK execution, and watch rebuilds through the same planner contract
• adapts existing document/image/audio strategies behind graph ArtifactTransforms instead of keeping a parallel rendering pipeline
• removes the legacy build dispatcher, dependency-map build path, file/template helpers, and graph-only config loader that became redundant
• preserves provider/capability evidence in frozen plans and uses the canonical artifact cache/executor path

Behavior

• exact targets and all-reachable selection now differ only in target selection, not execution architecture
• dry-run emits the exact serialized frozen ExecutionPlan while remaining side-effect free
• built-in capabilities participate without requiring a separate transforms file; configured transform edges remain additive
• v1 and v2 configs normalize through the same planner semantics
• SDK and CLI consume the same PlanningRequest → ResolvedExecution → execute lifecycle

Validation

• cargo fmt --all
• cargo clippy --workspace -- -D warnings
• cargo build --workspace --verbose
• cargo test --workspace --verbose
• bundled CLI/examples validation
• dedicated canonical planner tests for v1/v2 equivalence and dry-run purity

Closes #354