Replace the removed Zero endpoint mode option with readOnly

[Kn0ax]

The Zero endpoint examples still used the mode option, which @spacefast/zero 0.5.0 removed. This updates both examples and the endpoint() reference to readOnly. GET and HEAD endpoints are read endpoints by default, and every other method writes. readOnly overrides that default in either direction.

The zero_mutation_origin_invalid error page isn't in this PR. It's part of the producer-owned generated snapshot, which can't be split, so it ships with the reference refresh in #54.

Validation

verify:generated, check, validate, build, audit, verify:public-safety, and verify:routes pass with Bun 1.3.11 and Node 26.

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