Ship a loadable Height Tools example add-in

[protosphinx]

Why this matters

The add-in API documentation has an inline example, but contributors cannot download and load a checked-in working module. A small real example shortens the path from reading the API to making a first contribution.

Scope

• Add examples/addins/height-tools.mjs.
• Make its no-argument command work on the Starter plate by resolving exactly one height parameter and updating it through api.document.transact.
• Return a clear error if the parameter is missing or ambiguous.
• Link the example and loading instructions from the add-in documentation.
• Exercise the real example module from the existing add-in smoke instead of duplicating its implementation inside the test.

Relevant files

• examples/addins/height-tools.mjs
• docs/addin-api.md
• scripts/studio-addin-api-smoke.ts

Acceptance evidence

• Loading registers the manifest but grants no document access before approval.
• Enabling the add-in registers one namespaced command.
• Running the command changes the Starter plate height through a typed transaction.
• The exact rebuilt bounds and volume change by the expected amount.
• Disable and unregister remove commands and listeners.
• The documentation names the trust boundary: local add-ins are trusted browser code, not a sandbox.

Test commands

npm run build
npm run smoke:addin-api
npm run ci:gate

Contribution size

S: one example module, a documentation link, and focused lifecycle assertions.

Maintainer response expectation

Comment before starting. A maintainer will confirm scope and ownership before implementation; review will focus on the listed acceptance evidence.