Hooks: Add site-scoped callback registrations

[hokoo]

This adds native site-scoped callback registrations to the Hooks API while preserving global registrations as the default.

The existing registration, lookup, and removal functions receive an optional trailing $blog_id parameter:

• Registration accepts null for global scope or a positive integer for one site.
• Lookup and removal accept false for all scopes, null for global scope, or a positive integer for one site.
• Scope forms part of registration identity, allowing the same callback and priority to coexist globally and for multiple sites.
• Global and matching site-scoped callbacks retain their normal registration order.
• Scope is evaluated immediately before each callback, including nested dispatch and the all hook.
• The initial implementation intentionally supports scalar site IDs only.

Legacy global callback keys and callback array shapes remain unchanged. A separate global-only dispatch path avoids adding a per-callback scope check to existing hooks.

Tests cover global compatibility, matching and non-matching sites, identity and ordering, lookup and removal selectors, priority 0, nested and mid-dispatch changes, the all hook, reference-array variants, invalid IDs, preinitialized hooks, and adding the first scoped callback during dispatch.

Trac ticket: https://core.trac.wordpress.org/ticket/66097

Use of AI Tools

AI assistance: Yes
Tool(s): OpenAI Codex
Model(s): GPT-5
Used for: API design analysis, implementation, test development, performance benchmarking, and automated QA. The contributor selected and approved the public API decisions and is submitting the resulting patch.

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