Enforce non-zero context_id in registration writes

[starknetdev]

Summary

The registration component documents context_id == 0 as the "not registered" sentinel, but internal write paths do not reject context 0.

Affected Code

• packages/metagame/src/registration/registration_component.cairo:17
• packages/metagame/src/registration/registration_component.cairo:123
• packages/metagame/src/registration/registration_store.cairo:49
• packages/metagame/src/registration/registration_store.cairo:97

Risk

A host that uses context 0 can confuse real registrations with unknown tokens because _get_token_context() returns 0 for both. This may allow registration bypasses in consumers that check reverse lookup state.

Suggested Fix

Reject context_id == 0 in set_entry() and increment_entry_count().

Suggested Tests

Add negative tests proving:

• set_entry({ context_id: 0, ... }) reverts
• increment_entry_count(0) reverts