refactor(php-transformer): expose compact site plan result projection

[chubes4]

Summary

Continues #1360. Add TransformerResult::toCompactWordPressSitePlanView() to own the existing v1 projection plus v2 compaction sequence. SSI direct-artifact and URL-batch composition currently repeat that sequence.

The API lives on the result rather than ArtifactCompiler: callers retain the same composed result and its terminal-work metrics. Composition, validation, compact schema, and existing public methods are unchanged.

Verification

All ten checks passed on 1591a37abfd89e1c7975448841295339d4335f1d: Composer tests and WordPress integration across PHP 8.2-8.5, plus visual parity tools, and solved-site promotion.

php tests/contract/staged-artifact-compilation.php
composer run test:canonical
composer run test:parity

Run from php-transformer after installing dependencies. All passed; parity covers 307 fixtures. Contracts assert exact old/new compact-view equality for ordinary and reference-backed staged receipts, zero terminal payload reads, and retained zero-transform/normalization metrics on the same result.

SSI adoption follows the producer release; its direct-import checkpoint PR overlaps the consumer site and must be coordinated. No release or deployment included.

AI Assistance

OpenCode GPT-6 Astra coordinated implementation, review, and verification under Chris Huber direction. Independent review identified the terminal-metrics requirement; the final API preserves the result object rather than returning only a compact view from composition.