test: hand-coded MDN + WHATWG spec example tests

[chad-loder]

Summary

• Adds tests/test_spec_examples.py and tests/test_mdn_examples.py, exercising the canonical examples from the WHATWG URLPattern Standard and MDN.
• 23 new test cases, all passing against yarlpattern as-is. No production code changes.

Why these tests are separate from the WPT-corpus suite

The WPT corpus exercises 469 conformance cases by construction, but those entries are terse JSON tuples and not human-readable. The examples written into the prose of the spec and the MDN guide deserve a separate, narrated surface so that:

1. Regressions on the spec's headline examples surface as named failures.
2. Readers landing in the test tree see, in Python, exactly the patterns and inputs the standard authors used to illustrate the feature.

What's covered

tests/test_spec_examples.py (7 cases)

WHATWG URLPattern Standard §1.2 shop example:

http{s}?://{:subdomain.}?shop.example/products/:id([0-9]+)#reviews

• 3 positive matches (with and without subdomain, both schemes)
• 4 negative matches: missing #reviews, explicit port disallowed, search disallowed, non-digit id

tests/test_mdn_examples.py (16 cases)

• "Using multiple components": kitchen-sink protocol+username+password+hostname+pathname pattern with full-population and zero-optional input matches.
• "Custom prefix and suffix modifiers": {:subdomain.}*example.com hostname wildcard with named-group expansion across 0–3 labels, plus 3 negative cases (leading dot, wrong eTLD, trailing dot).
• "Pattern syntax — regex groups": /books/:id(\d+) digit-only constraint with 3 positive cases and 4 negative cases (letters, trailing junk, empty id, signed integers).

Test plan

• uv run pytest tests/test_spec_examples.py tests/test_mdn_examples.py — 23 passed
• uv run pytest -q — 603 passed, 19 skipped (was 580 before)
• just lint — all green (ruff, ty, pyright, semgrep, shellcheck, rumdl, codespell, interrogate, validate-pyproject)
• Signed commit (ED25519)

Part of the v0.2.0 roadmap.