Harden bounded site artifact compilation

[chubes4]

Summary

• negotiates bounded compiler limits and emits precise WordPress site-plan validation diagnostics
• preserves explicit sibling routes and safely canonicalizes link, artifact-path, and encoded asset references
• indexes repeated artifact-path, selector, and stylesheet work to reduce large-batch compiler CPU
• retains current trunk HTML conversion and parity behavior while adding the artifact-scale fixes required by Compile shared URL-import resources once across resumable batches static-site-importer#771

Scale evidence

This branch was exercised through Studio against https://www.richlynngroup.com/ as part of the dependent SSI URL collector run:

• 1,145/1,145 discovered routes completed
• 124/124 bounded batches completed
• retained batches resumed across encoded-path and route-collision failures
• retained batch compiler user CPU decreased from about 415s to 285s (about 31%)
• peak RSS remained bounded near 822 MB for the measured 115.7 MB artifact

The run proves the compiler can participate in the full large-site workflow. It does not claim visual/editor perfection: the generated Richlynn evidence site still contains HTML fallbacks and is not accepted under SSI's zero-fallback parity gate.

Validation

• composer test
• 258 PHP transformer parity fixtures
• canonical contracts, unit tests, and package install proof

Dependencies

• SSI collector: Support resumable large URL site imports static-site-importer#798
• Studio orchestration: Add studio site create --from static source imports studio#3952

AI assistance

• Model: openai/gpt-5.6-sol
• Tool: OpenCode
• Used for: codebase investigation, artifact compiler profiling, implementation, merge-conflict resolution, regression coverage, large-site verification, and PR drafting. Chris Huber remains responsible for the submitted changes.