Extract atomic JSON Patch package with stable arrays and conformance tests

[dermatthes]

The deterministic patch engine currently lives in AI Harness, which prevents using it independently of AI and schema dependencies. This PR extracts it into phore/json-patch, under Phore\JsonPatch\.

• Moves 12 core classes and the two existing core test files from Implementierung: atomare Struct-Patches mit stabiler Array-Adressierung phore-ai-harness#6; only their namespaces change.
• Adds PHP 8.5 / Composer package configuration, documentation and three executable examples covering ordinary patches, atomic failure/conflicts and combined stable-array edits.
• Compares json-patch-tests, python-json-patch and fast-json-patch tests. Includes the pinned, unchanged Apache-2.0 shared corpus with attribution/license and 75 independently written regression data sets. docs/test-coverage.md maps gaps to tests and documents deliberate differences.
• Adds CI for Composer validation, PHPUnit and executable examples.

Validation: PHP 8.5.10 / PHPUnit 13.3.3: 200 tests, 628 assertions, 4 cases disabled upstream. Two disabled semantic cases are covered by enabled independent regressions; duplicate-member JSON syntax remains outside the native decoder. All three examples pass; composer.json validates strictly. No runtime fixes were required by the additional cases.

Runtime has no AI/schema dependencies. Existing strict defaults and typed-operation field rules are preserved and documented. Source is prepared for later publication; this PR does not tag or publish the package. Dependent Harness draft: phore/phore-ai-harness#8.

GitHub Actions verified on this commit: https://github.com/phore/phore-json-patch/actions/runs/34536649427 — Composer validation/install, 200 tests / 628 assertions / 4 upstream skips, and all three executable examples pass.