v2.0.1: PHPStan bug sweep

Internal hardening release. No functional changes.

Fixes

• Fix block_categories_all filter signature mismatch (callback registered with accepted_args=2 but only declares 1 parameter)
• Fix esc_attr() type errors when emitting external asset tags — cast int post_id and int|false filemtime result to string before passing
• Properly escape stylesheet id attribute in external file output
• Remove redundant is_array() / is_string() checks against already-narrowed types

Verification

• PHPStan level 5 passes with 0 errors on the entire plugin
• All previously broken features verified working