refactor: simplify executor materialization and fallback logic

[mvallebr]

This PR refines Synaflow's materialization model by pushing more planning into the compiled Dag and simplifying what the executors decide at runtime.

What changed:

• resolve more materialization semantics at build time
• centralize eager-materialization planning in Dag helpers instead of re-deriving it in sync/async executors
• keep resolved materializer callables and consumer materialization metadata as part of the compiled contract
• fix PEP 563 materializer-context regressions so runtime factories receive the resolved item type
• add/adjust tests around build-time planning, runtime materialization behavior, and PEP 563 parity
• trim executor tests that became redundant after the planning logic moved into Dag
• update docs to reflect the stronger build-time vs run-time separation

Why:

• reduce duplicated materialization logic across executors
• make the Dag a clearer execution contract for custom runners and future runtime overrides
• preserve sync/async parity while keeping runtime behavior simpler and more predictable

Validation:

• uv run pytest