Cut and publish 1.0.0.0 release to Hackage

## Summary

Tag and publish `siren-json-1.0.0.0` to Hackage, closing this milestone.

## Motivation

Closing event for the milestone — all the modernisation work is in service of this release. Once published, `siren-json-1.0.0.0` is the public-facing artifact users install.

## Scope

- Open a release-prep PR:
- Confirm `version:` in `siren-json.cabal` is `1.0.0.0` (set as part of #62).
- Move CHANGELOG `Unreleased` content to `## [1.0.0.0] — YYYY-MM-DD`.
- Open new empty `Unreleased` section.
- Merge release-prep PR.
- Tag: `git tag -a v1.0.0.0 -m "siren-json 1.0.0.0"` and `git push --tags`.
- Verify the release workflow (#64) produces a candidate at the expected URL.
- Visually inspect the candidate on Hackage: metadata, Haddock rendering, dependency bounds.
- Promote candidate to published release via the manual workflow job.
- Create a GitHub release pointing at the tag with the CHANGELOG section as the body.

## Acceptance criteria

- [ ] `v1.0.0.0` tag exists on `main`.
- [ ] `https://hackage.haskell.org/package/siren-json-1.0.0.0` is live.
- [ ] Haddock published (visible on Hackage, not "haddock build failed").
- [ ] GitHub release created with CHANGELOG body.
- [ ] Milestone closed.

## Additional context

Blocked by #61, #62, #63, #64, #65 minimum. Stackage submission is a deliberate follow-up (out of milestone) — file when there is user demand.