A stale build/ left by composer zip is gitignored output, but phpcompat still
scanned it, so composer test could fail on a copy of the plugin rather than on
the source. phpcs already excluded it; this makes the two agree.

Server-side mode renders blocks over 128KB as plain text rather than
highlighting them, and nothing in the advanced guide said so. Add a Large
code blocks section covering the threshold, the wzcbh_server_highlight_max_bytes
filter and disabling server-side highlighting with a zero limit, and note the
exception where the guide claims both modes look identical.

spl_object_hash() is deprecated as of PHP 8.6. spl_object_id() carries the same
guarantees the callback key relies on: unique for the object's lifetime, and
reused only after it is freed. The key already includes the class name, so hook
identity and add/remove behaviour are unchanged.

Options_API::update_option() and ::delete_option() default the
get_option() read to an empty array, which covers a missing option but
not one that exists holding something other than an array. Writing an
offset to a string is an uncaught TypeError on PHP 8, so a settings
option corrupted by a bad migration, a manual database edit or an
option filter takes the site down on the next settings write.

Fall back to an empty array when the stored value is not one.

Add a core/code deprecation matching code-syntax-block markup and stop the registerBlockType filter from overriding deprecation save functions.