NAP-04: Stream current context request topic

[dskvr]

Summary

• Adds NAP-04 for the stream:current-context-get IFC topic.
• Defines an optional requestId correlation payload for requesting current stream context.

Why

• Stream context discovery needs explicit request semantics without bundling the response topic into the same proposal.

Spec / API surface

• Discovery: shell.supports("ifc", "NAP-04").
• Manifest dependency remains ["requires", "ifc"].

Compatibility

• Existing empty-payload request behavior remains valid; requestId is optional.

Validation

• git diff --cached --check passed before commit.
• Checked for private implementation references in the spec and README row.

Non-goals

• Response payload shape, shell focus/window policy, livestream channel switching, and generic IFC transport.

Follow-ups

• NAP-05 covers the companion stream:current-context response topic.

Supersedes the stream:current-context-get portion of the closed umbrella topic payload PR #20.

[dskvr]

Closing in favor of #26, which keeps stream:current-context-get with the related stream current-context and channel-switch semantics in one stream:* family NUB.