Follow-up to PR #350 review:
1. New assertion: hyperhive.frontend.extraFiles[*].target must be a
relative path inside the static dir — leading '/' and '..'
segments rejected at config eval time. Belt-and-braces against
string-concat-into-paths escapes (the boundary doc flags this
pattern even though agent.nix goes through operator review).
2. Documented overwrite semantics in the option doc: collision with
a default-dist path or with a prior entry's target is a hard-fail
(`refusing to overwrite existing path …`). To override a default
file, fork `hyperhive.frontend.dist` instead — extraFiles is
pure additions.
The collision-hard-fail behaviour was already implemented in
`mergedDist` (in commit a19e156); this commit just makes the
contract explicit in the docstring.
Refs #273, addresses damocles' notes on PR #350.
2951da32e7