Harden php-transformer canonical contracts

[chubes4]

Summary

• Tighten canonical result validation for required top-level keys, status values, report placement, conversion report schema/source format, and materialization plan schema/shape.
• Make MaterializationPlanBuilder::fromResult() validate canonical envelopes and return the canonical embedded plan instead of silently rebuilding or accepting partial arrays.
• Remove product-shaped products output from the canonical materialization plan while keeping source manifests available as generic artifacts/assets.
• Update contract coverage and the result envelope docs so legacy_mapping is rejected rather than documented as required/transitional.

Verification

• composer validate
• composer test
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Auditing php-transformer canonical contracts, implementing validation/test/doc cleanup, and preparing the PR.