Add dynamic {aql:} placeholder tokens for query values

[ryanwelcher]

Summary

Adds dynamic placeholder tokens of the form {aql:name} that can be used in any query value, most usefully as a meta query value. Tokens are resolved centrally on the raw block query params before any trait or the aql_query_vars filter runs, so every control (built in or third party) supports them without per-control code.

• Placeholder_Resolver (includes/Placeholder_Resolver.php). A pure class that reads only the context array passed to it, never global WP state, so it is fully unit-testable. Tokens that resolve to nothing are left in the value verbatim. A logged-out {aql:user_id} therefore compares against a literal string that matches no real meta value, so the clause matches nothing rather than widening to match everything.
• Resolved in all three query paths. Frontend inherited queries, frontend non-inherited queries via query_loop_block_query_vars, and editor previews via rest_{post_type}_query. The editor sends the edited post ID with preview requests (aql_preview_post_id) via an apiFetch middleware so post-relative tokens resolve correctly in the block editor.
• Built-in placeholders. Current post, parent, author, logged-in user, current term on archives, and a family of date and time tokens (current_date, current_date_compact for ACF Ymd dates, current_datetime, current_time, current_timestamp, zero-padded date parts, and date_minus_* / date_plus_* offsets). All use the site timezone.
• Two extension filters. aql_resolve_placeholder resolves a custom token at query time and aql_placeholder_list lists it in the editor picker.
• Editor picker. The Meta Value field becomes a single-value token field that lists placeholders on focus with label and description. PlaceholderTextControl is exported on window.aql so custom SlotFill controls can reuse it.
• Docs. New "Dynamic placeholders" section in extending-aql.md covering the token table, recipes (upcoming ACF events, running events, relationship fields), custom registration, and a caching note. Readme entries added.

Testing

• npm run test:unit on this branch: 205 tests, 340 assertions, all passing. tests/unit/Placeholder_Resolver_Tests.php covers every built-in token, unresolved-token passthrough, nested arrays, and the filter hooks.
• New Playwright spec tests/e2e/tests/dynamic-placeholders.spec.ts covers the picker in the Meta Value field. Not run in this session (needs a wp-env instance).

Related

#179 (meta query modal) is stacked on this branch and should merge after it.