docs(scheduler): add terminology note distinguishing ContentFidelity (memory) from ContextFidelity (CAM spec)

[bug-ops]

Description

CAM spec (specs/062-context-adaptive-memory/spec.md) introduces a new ContextFidelity enum in zeph-common/src/fidelity.rs (to be created in implementation phase). There is already an existing ContentFidelity enum in crates/zeph-memory/src/optical_forgetting.rs for ScrapMem optical forgetting.

The two enums are semantically distinct (the spec correctly notes "zeph-memory — no changes"), but they have near-identical names and identical variant names (Full, Compressed, Placeholder vs Full, Compressed, SummaryOnly), which will cause confusion for implementors.

Recommendation

Before implementing CAM (#4017, #4016, #4018):

1. Add a module-level doc comment to optical_forgetting.rs explicitly distinguishing ContentFidelity (long-term memory store fidelity) from the forthcoming ContextFidelity (context-window fidelity from CAM spec).
2. Consider whether to rename one of them before the CAM implementation lands to prevent future confusion.

Environment

• Version: HEAD 53f8f05
• Relevant files:
  • crates/zeph-memory/src/optical_forgetting.rs — existing ContentFidelity
  • specs/062-context-adaptive-memory/spec.md — forthcoming ContextFidelity