Replace retired Splines demo with a local Oxford HAL example

[dblock]

Summary

Closes #67 by making a reproducible local Oxford demo the main example instead of the retired Splines Heroku app.

• Add a read-only localhost HAL server and runnable Hyperclient client demonstrating collections, attributes, relative link traversal, embedded file metadata, templated links, and CURIEs.
• Use a small factual subset of the archived Balliol College record, retaining its original place paths. Document exact snapshot provenance and demo adaptations; no images are downloaded or redistributed.
• Replace Splines README usage examples and remove the obsolete Splines script. Keep generic write-method documentation, explicitly noting the local demo rejects writes.
• Add automated offline smoke coverage to the existing Minitest suite and WEBrick as a development/test dependency, not a runtime dependency.

The underlying OxPoints place data is CC0: Oxford explicitly states "Public domain (CC0; no attribution required)" on slide 8 of its 2011 OxPoints presentation. The demo README documents this primary-source licensing evidence. This does not claim a license for complete archived API responses or photographs; no images are redistributed. Collection/template/CURIE endpoints are explicitly documented as newly authored demo additions. The dataset is historical and intentionally minimal, not a mirror of the retired service.

Validation

Full Minitest and Spinach suites and RuboCop pass. Also started the actual WEBrick server on a local ephemeral port, verified its HAL response, and ran the documented client successfully over HTTP, then stopped the server.

The pre-existing untracked examples/README.md is untouched and excluded. This PR is independent of repository-hygiene PR #327.

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coverage: 99.687% (-0.3%) from 100.0% — dblock:dblock/oxford-example into codegram:master