NAP-NN: Topic payload semantics over IFC

[dskvr]

Summary

• Adds NAP-NN-TOPIC-PAYLOADS.md, a provisional NAP-NN draft for IFC-carried topic payload semantics.
• Updates the README draft table to list the new profile/chat/stream topic payload protocol.

Why

NAP-IFC defines the generic inter-frame transport: ifc.emit, ifc.subscribe,
ifc.unsubscribe, and ifc.event. It does not define what a topic such as
profile:open means.

This PR makes those topic payloads public napplet-agreed semantics instead of
leaving them as implementation-local conventions.

Spec / API surface

The draft defines payload and producer/consumer behavior for:

• profile:open
• chat:open-dm
• livestream:channel-switch
• stream:current-context-get
• stream:current-context

The draft references NAP-IFC as the carrier and does not redefine IFC envelopes
or add a new shell API.

Compatibility

• Existing NAP-IFC transport remains unchanged.
• Consumers are expected to ignore malformed payloads, unknown fields, and
  unsupported topics.
• The provisional NAP-NN-TOPIC-PAYLOADS identifier should be replaced with
  the assigned NAP-NN number after merge.

Validation

• git diff --check -> PASS
• git diff --check HEAD~1..HEAD -> PASS
• rg -n "hyprgate|napplet/napplet|@napplet" NAP-NN-TOPIC-PAYLOADS.md README.md -> no matches
• git status --short --branch -> ## nap-topic-payload-semantics...origin/master [ahead 1]

Non-goals

• Shell window management, workspace placement, or focus policy.
• wm:* shell telemetry topics.
• shell:* host command topics.
• Deprecated identity topics such as auth:identity-changed.
• Stale stream:channel-switch alias promotion.
• Media playback handoff; that remains NAP-MEDIA / NAP-PLAYER territory.

Follow-ups

• Assign the final NAP-NN number during review/merge.
• Update downstream SDK/docs references if reviewers rename the draft or change
  payload fields.
• Revisit typed helper ergonomics only after the topic protocol shape settles.

[dskvr]

Closing this umbrella topic-payload PR because the topic semantics should be split into separate numbered NUB-NN drafts per topic instead of one combined NUB. I will replace this with one PR per topic.