Content patches: CND-side comprehension — validate sweep, docs matrix, CND-diff scaffolding (P5)

[romain-pm]

Sub-issue of #725 (P5). Addresses the second review point on #697: the framework does not, by itself, make it easier on the CND side to understand the migration process. With the engine in Java (#697), the answers are language-neutral and serve Java and JS modules alike.

Scope

• Docs matrix: a guide section mapping each CND change to the operation it requires — property dropped → removePropertyValues, property added with a default → setPropertyValues, property type changed → convertPropertyValues, type retired → removeNodeType, type renamed → changeNodeType — and, just as important, which CND changes need no patch. Can start immediately.
• validate sweep: a content-vs-definitions divergence report in the Java framework (per module: leftover values of properties no longer defined, instances of unregistered types, stored values whose type no longer matches the definition), exposed through the P4 GraphQL/CLI surfaces (content-patches validate).
• CND-diff-aware scaffolding (candidate): extend P4's create scaffolding to read the definitions diff between the deployed and the local CND and suggest the patch skeleton.

Dependencies

• Blocked by feat: content patches — run-once content transformations in JavaScript #697.
• The validate exposure is additionally blocked by the admin API (Content patches: admin GraphQL API — status, reset, run (P4) #728); the CND-diff scaffolding builds on the optional CLI's create (Content patches: developer CLI — status, run, reset, create (optional, split from P4) #731). The docs matrix has no blocker.