feat: add RFC 3986 relative IRI resolution

[soyuka]

Summary

• Adds IriHelper::resolve() implementing RFC 3986 §5 relative reference resolution
• Needed because CSS JSON-LD responses may contain relative @id values
• Handles all RFC 3986 edge cases: dot segments, authority-relative, query/fragment

Stack

4/8 — builds on #8 (feat/jsonld-parsing)

Test plan

• Full RFC 3986 §5.4 normal examples via data provider (22 cases)
• Solid Pod-specific relative ID resolution tests
• Absolute reference passthrough test

🤖 Generated with Claude Code

[soyuka]

CSS (Community Solid Server) returns JSON-LD with relative @id values when you GET a container with Accept: application/ld+json. For example, GETting http://pod.example/data/:

  {
    "@id": "./",
    "@type": ["http://www.w3.org/ns/ldp#BasicContainer"],
    "http://www.w3.org/ns/ldp#contains": [
      { "@id": "file.txt" },
      { "@id": "sub/" }
    ]
  }

The @id values "file.txt" and "sub/" are relative references — they're not usable as-is. To construct the actual resource URLs (http://pod.example/data/file.txt,
http://pod.example/data/sub/), you need to resolve them against the base URL of the request.

getContainerContents() (f259316) uses IriHelper::resolve() on every @id from the JSON-LD response to turn relative references into absolute URLs that can be used for subsequent requests (HEAD, GET,
DELETE, etc).

Without it, container listing would only work when CSS happens to return absolute URLs — which it doesn't by default.