feat: relevanssi excerpt in search queries

[batonac]

closes #4

Tested in production and can be seen live here https://nordenmfg.com/?s=norden

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Note

Medium Risk
Changes excerpt generation logic and global $post handling for REST query responses, which can affect search result formatting/highlighting and content returned by the API.

Overview
Search queries using Relevanssi now return the plugin-generated excerpt (including highlighting) through the query REST API’s dynamic postexcerpt field instead of always regenerating/stripping excerpts.

To preserve Relevanssi data, process_specific_dynamics() accepts an optional post object and callers pass the WP_Query post through; excerpt output now conditionally returns raw highlighted HTML (no character limit) or a tag-stripped truncated version (with character limit).

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