P4: Raise coverage threshold to 80% and add key-path tests

[SisyphusZheng]

What to build

Coverage is currently 70.9% with a 70% threshold. Raise the threshold to 80% and add tests for the most important untested runtime paths.

Acceptance criteria

• deno.json:78 threshold updated to 80.
• Tests added for core/csr, core/dsd-hydration, adapter-vite/cli/build-ssg, ssg/postprocess.
• deno task test:coverage:check passes.

Blocked by

#287