[SE-3-04] Portal page structure validator

[DeveloperAlly]

Tracking issue for SE-3-04 in the Sprint 3 + Post-Migration Audit stream.

Scope:

• Implement Portal page structure validator
• Keep changes isolated to the task branch/worktree
• Validate against docs-v2 governance rules before PR

[DeveloperAlly]

Validated on docs-v2 on March 11, 2026. Closing as implemented by tools/scripts/component-layout-governance.js, with portal pages covered through tools/config/component-layout-profile.json path matching and structure rules.