Content patches declared in JavaScript

[romain-pm]

Part of EPIC #554. Implemented — under review in #697.

registerContentPatch: versioned, run-once content transformations executed at module start, backed by core's patch status store (persistent halt-on-failure barrier), with guard-railed operations (patch.*) and low-level jcr.* access. Design and decisions: ADR-0006, CONTENT-PATCHES-PLAN.md. Live-verified on Jahia 8.2.3 (incl. getRealNode().setPrimaryType() retype handling and property remapping around retypes).

Docs: guide 6-content-patches. Tests: registrar unit tests + contentPatchTest.cy.ts.