fix: align fixed range-mode behavior across context, pipeline, doctor, and docs

[chtnnh]

Problem

The context resolver supports rangeMode: "range" without an active session, but pipeline, strict doctor, and ship treat that documented configuration as broken.

Scope

Choose one coherent fixed-range contract and apply it consistently.

Acceptance criteria

• Fixed range mode either works without a session everywhere or is rejected/documented everywhere.
• status, doctor --strict, ship, and the resolver agree.
• Documentation describes the same required setup.
• Tests cover the supported no-session path and the rejected path, as applicable.