Add visible corner-relief authoring for sheet metal

[protosphinx]

Why this matters

Rectangular and circular corner reliefs already have a typed lifecycle, schema validation, exact kernel execution, and tests, but they remain unavailable from the visible ribbon.

Scope

• Add a visible command for selecting a base flange, eligible corner, and relief style.
• Dispatch only sheetMetal.cornerRelief.create.
• Show the derived relief size bendRadius + thickness as read-only evidence.
• Reuse the existing sheet-metal deletion route.
• Keep non-perpendicular reliefs and per-flange size overrides out of scope.

Relevant files

• src/page.html
• src/static/studio.js
• src/static/studio-v6-ui-registry.js
• scripts/studio-sheet-metal-smoke.ts
• docs/sheet-metal.md

Acceptance evidence

• An ordinary mouse and keyboard flow creates a corner-relief record without direct document mutation.
• The feature is inserted before dependent edge flanges.
• The exact volume decreases by the documented rectangular or quarter-disc amount.
• Canonical save and reopen preserve the relief.
• Invalid or duplicate corners leave the revision unchanged and show a useful error.
• The resulting solid remains B-rep valid with complete unique face, edge, and vertex names.

Test commands

npm run build
npm run smoke:sheet-metal
npm run smoke:ui-foundation
npm run smoke:browser
npm run ci:gate

Contribution size

M: one bounded visible workflow over an existing typed and exact kernel capability.

Maintainer response expectation

Post the short UI or fixture approach requested above and wait for maintainer scope confirmation before implementation. Review will focus on the listed acceptance evidence.