usubscription: explicitly require SubscriberInfo.uri.resource_id = 0 for subscriber identity

[PLeVasseur]

Problem

During review of eclipse-uprotocol/up-streamer-rust#77, we hit ambiguity around subscriber URI examples used in subscription data (for example //authority-b/5678/1/1234).

Reviewer feedback suggested using resource 0x0000 for subscriber UUris for clarity. That appears consistent with endpoint semantics, but the current uSubscription text does not explicitly require it.

Relevant spec context

• SubscriberInfo is defined as subscriber identification (URI of the subscribing uEntity):
  • https://github.com/eclipse-uprotocol/up-spec/blob/main/up-core-api/uprotocol/core/usubscription/v3/usubscription.proto#L120-L124
• FetchSubscriptions by subscriber is defined as subscriber -> topics:
  • https://github.com/eclipse-uprotocol/up-spec/blob/main/up-l3/usubscription/v3/README.adoc#L242-L245
• Subscriber URI validity currently requires non-wildcard authority/ue_id/resource_id, but does not require resource 0:
  • https://github.com/eclipse-uprotocol/up-spec/blob/main/up-l3/usubscription/v3/README.adoc#L264-L273
• Endpoint-style addressing in uProtocol consistently uses resource_id = 0:
  • notification sink: https://github.com/eclipse-uprotocol/up-spec/blob/main/basics/uattributes.adoc#L263-L266
  • request source: https://github.com/eclipse-uprotocol/up-spec/blob/main/basics/uattributes.adoc#L293-L296
  • response sink: https://github.com/eclipse-uprotocol/up-spec/blob/main/basics/uattributes.adoc#L384-L387
• By contrast, publish topic resources use [0x8000, 0xFFFE]:
  • https://github.com/eclipse-uprotocol/up-spec/blob/main/basics/uattributes.adoc#L213-L216

Gap

There is no explicit normative statement that SubscriberInfo.uri.resource_id must be 0, which leads to inconsistent examples and reviewer confusion.

Proposed change

Add explicit uSubscription requirement text that for SubscriberInfo.uri:

• resource_id MUST be 0 (subscriber endpoint identity),
• existing non-wildcard requirements remain in force.

Also update:

• up-l3/usubscription/v3/README.adoc (normative text + examples),
• up-core-api/uprotocol/core/usubscription/v3/usubscription.proto comments for SubscriberInfo.

Why

This removes ambiguity, aligns SubscriberInfo with established endpoint semantics, and improves interoperability/readability across implementations and tests.