manifest.yaml gains template_overrides [{template, block}] and
email_wrappers [system keys]; sources by convention at
templates/overrides/<template>/<block>.ninjatpl and
templates/email/<system>.ninjatpl (validated at build). Populates the
existing manifest fields BlockTemplateOverrides / EmailWrapperSystemKeys —
no proto change. Unblocks full theme-catalog codeless conversion: every
theme in the fleet registers overrides + an email wrapper.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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