Track upstream Site Editor preload fix for Studio performance

[chubes4]

Summary

Track the upstream Gutenberg Site Editor preload fix and verify its impact on Studio's Site Editor load metric once available in the bundled WordPress/Gutenberg path.

Upstream links

• Gutenberg issue: Site Editor: preload template parts, query posts, and taxonomies used during initial canvas load WordPress/gutenberg#78074
• Gutenberg PR: Site Editor: preload initial canvas data WordPress/gutenberg#78075
• Homeboy-rigs durable harness: Add Studio Site Editor preload comparison workload chubes4/homeboy-rigs#106

Why this matters for Studio

Studio/Playground makes individual WordPress REST requests relatively expensive. The Site Editor currently issues several REST requests during initial canvas load that can be known and preloaded server-side:

• individual template part records
• Query Loop post collection requests for per_page=10 and per_page=3
• public taxonomies

The upstream fix should reduce the number of measured REST resources during the Studio Site Editor load path.

Current evidence

Homeboy-rigs Site Editor preload comparison harness:

baseline_site_editor_measure_ms: 1235
candidate_site_editor_measure_ms: 914
site_editor_preload_delta_ms: -321
site_editor_preload_delta_pct: -26.0
baseline_measure_resource_count: 5
candidate_measure_resource_count: 0

Prior smoke on the same candidate shape also showed ~500ms / ~35% improvement.

Done when

• Upstream Gutenberg PR lands or an equivalent Core sync lands.
• Studio's WordPress/Gutenberg bundle includes the fix.
• The Studio Site Editor load benchmark is rerun and confirms the missed REST resources no longer appear in the measured path.