Validate namespaced provider SVG gradients

[konojunya]

Summary

• allow safe local linear and radial gradients in processed provider SVG
• require gradient IDs to use a collision-resistant stack- namespace
• reject missing, duplicate, unused, external, executable, or structurally invalid references
• add namespace-identifiers to the visual-preservation transformation log
• keep stylesheets forbidden after import

Why

The audited Azure icon archive uses local SVG gradients. Namespacing referenced IDs preserves the official artwork while preventing collisions when several icons are embedded into one standalone diagram.

Validation

• npm run validate
• npm run generate:check
• npm test
• npm run typecheck
• cargo fmt --all --check
• cargo test --workspace --locked
• git diff --check