docs(readme): consolidate conformance, surface subtle correctness wins

[chad-loder]

Summary

Restructures the README so the conformance story is the first H2 after the hero code, replacing the older "WHATWG conformance" + "Test corpus matrix" + "API surface" split which fragmented the same story across three sections.

Depends on #21 — the new conformance table claims 469 / 469 + the in-repo 130 unit tests = 599. That's only true once URLPattern.generate() lands. Either merge #21 first, or merge this and #21 together.

What changes

New ## Conformance section (right after the hero code)

• Headline: 469 / 469 upstream WPT pass + reproducibility blurb pointing at the SHA-pinned corpus.
• Single seven-row table covering all five WPT suites plus the in-repo unit-test suite (599 total).
• Two prominent links:
  • Full per-case conformance report
  • Documented deviations and stricter-than-yarl rules

New ### What we get right that's easy to miss subsection

Promotes the per-component canonicalisation rules that distinguish a spec-strict implementation from a stdlib approximation:

• WHATWG URL parsing end-to-end via yarl (not urllib.parse)
• IDNA2008 / UTS46 hostname canonicalisation via the third-party idna package (not Python's stdlib IDNA2003 codec)
• Strict port parsing — "8080xyz" rejected
• Case-preserving %XX in pattern literals (WPT 146 / 148)
• U+FFFD substitution for unpaired surrogates (WPT 157)
• Hostname truncation at ? / # / / / \

Consolidations

• Dropped the duplicate "three places stricter than yarl" list from the How this differs from yarl section in favour of a one-line pointer to the new Conformance section + SPEC_DEVIATIONS.md.
• Folded the API-surface table into a single sentence — the table was redundant alongside the WPT-suites table since every row was green.
• Tightened the Install and BYO-regex-engine sections so the 99.5% / 100% framing lives in exactly one place (the Conformance section's stdlib-only callout).

Test plan

• just lint — all tools green (ruff, mypy, pyright, ty, semgrep, shellcheck, rumdl, codespell, interrogate, validate-pyproject)
• Headings outline reads cleanly: Hero → Conformance → aiohttp comparison → yarl comparison → Install → BYO regex engine → Quick start → Architecture
• Hatch's PyPI-substitution rules in pyproject.toml rewrite the new relative links (docs/wpt-compliance.md, SPEC_DEVIATIONS.md, scripts/fetch_references.sh) to absolute GitHub URLs at sdist build time
• Signed commit (ED25519)

No behavior changes; README-only.