Write Safely (No Clobbering)

Goal: Upload an object's bytes to a path. By default, fail loudly if the destination already exists rather than silently overwriting.

When to use this: You're writing something new and the destination should not exist yet, OR you're updating an existing object and want to verify it hasn't changed since you last looked.

Recipe — new file, refuse to clobber

Pass if_dest: { "kind": "fail" } to refuse to overwrite. The call fails with AlreadyExists if the address is taken.