Normalize subscriber fixture UUris to resource_id=0 for clarity

## Context

In PR #77 review, a comment pointed out that subscriber UUris in test fixtures currently use non-zero resources (for example `//authority-b/5678/1/1234`).

For subscriber endpoint identity, using resource `0` is clearer and aligns with uProtocol endpoint-style addressing conventions.

Related spec clarification issue:
- eclipse-uprotocol/up-spec#321

## Problem

Current fixture data can be read as if subscriber URI resource represents a topic/resource endpoint, which is confusing when the object is `SubscriberInfo` identity.

## Proposed change

- Update subscription-related fixtures/tests to use subscriber UUris with `resource_id = 0`.
- Keep publisher/topic resource semantics unchanged (event topics remain in publish range).
- Update any related test assertions/docs to match the normalized fixture values.

## Scope candidates

- `up-streamer/src/routing/subscription_cache.rs`
- `up-streamer/src/routing/subscription_directory.rs`
- `up-streamer/src/routing/publish_resolution.rs`
- `up-streamer/src/data_plane/ingress_registry.rs`
- Any other tests/examples using subscriber fixture URIs with non-zero resource IDs.

## Why

This is a clarity/readability improvement that reduces ambiguity in tests and review discussions, and aligns local fixtures with the direction proposed in the upstream spec clarification.