[codex] Add materializer execution overrides

[mvallebr]

Summary

This is the first runtime-DI slice for Issue 45.0.

It introduces ExecutionOverrides focused on materializers only, while keeping the DAG as the compiled semantic contract.

What changed

• added synaflow.execution.ExecutionOverrides
• added PipelineRegistry and MaterializerRegistry with strict key validation against compiled materializer keys
• threaded overrides through run(), async_run(), PipelineExecutor, and AsyncPipelineExecutor
• resolved materializers at runtime from the registry without re-deriving DAG semantics
• kept eager vs lazy materialization planning in the compiled Dag
• added sync/async tests for override behavior and fail-fast validation
• documented the new runtime-override boundary in README and build-vs-run docs

Why

PR #46 moved more materialization planning into the compiled Dag, which made the next design step cleaner: allow runtime replacement of concrete materializer callables without letting runtime mutate graph structure or execution semantics.

This PR keeps that boundary explicit:

• the Dag still decides topology, dependency resolution, and materialization strategy
• ExecutionOverrides only swaps the concrete callable associated with an already-compiled key

Impact

Tests can now replace compiled materializers without patching module globals or rebuilding alternative pipeline definitions.

This is intentionally scoped to materializers first. Observers/resources and richer scope helpers can build on the same contract later.

Validation

• uv run pytest