codegen/cli: regeneration rewrites unchanged files, --bindgen-only help names a file never produced, cli.mdx name/key claim, vite-plugin-demo uses a removed icp-cli step

[b3hr4d]

Smaller findings from the codegen tooling second pass (#660–#665).

1. Codegen rewrites every file on each run, even when the bytes are identical. Watchers such as tsc --watch, a bundler or an IDE wake on every regeneration, and the vite plugin's HMR sees changes that aren't there. Writing a file only when its content differs fixes this. Keep the temp-file + rename write chosen for symlinked outputs.
2. --bindgen-only's help text names a .did.d.ts file that is never produced.
3. docs/.../cli.mdx says a canister's name must match its key, but nothing enforces that, and --canister looks up the key. Either enforce it in loadConfig, with an error naming both, or correct the docs. Correcting the docs is the smaller change.
4. examples/vite-plugin-demo/icp.yaml uses the assets sync step, which icp-cli 1.5 removed. CI pins icp-cli 1.2.0, so CI still passes, but a user on current icp-cli can't deploy the example. This needs a decision on which icp-cli version the examples target.

Acceptance

• Regenerating with no input change leaves every output file's mtime unchanged. The test fails on main.
• The help text and cli.mdx match the code.