feat: add pipeline execution timing and performance tracing

[Copilot]

Adds per-step and per-phase execution timing to the pipeline, surfacing performance data through the existing tracing infrastructure without new dependencies.

Changes

PipelineStep trait (step.rs)

• Added name() -> &str with default "PipelineStep"; concrete steps override for actionable log output

StrategyStep (strategy_step.rs)

• Overrides name() → "StrategyStep"

TransformRegistry::apply_all (registry.rs)

• Times each transform with Instant; logs duration_ms at DEBUG level as a structured field alongside the existing transform field

Pipeline (pipeline.rs)

• run_steps: times each step, logs name + duration_ms at DEBUG
• run: times the transform phase, step phase, and total pipeline (reusing transform_start as the pipeline epoch to avoid measurement gaps); logs each at DEBUG

Observability

Timing is visible via existing CLI flags — no config changes needed:

# per-step + per-phase timing
renderflow --verbose build --config doc.yaml

# also exposes TRACE-level transform internals
renderflow --debug build --config doc.yaml

Example log output at DEBUG:

DEBUG transform=EmojiTransform duration_ms=0 Transform completed
DEBUG transform=VariableSubstitutionTransform duration_ms=1 Transform completed
DEBUG duration_ms=2 Transform phase completed
DEBUG step=StrategyStep duration_ms=340 Pipeline step completed
DEBUG duration_ms=342 Pipeline execution completed