docs(readme): rewrite top-level README for hydrologists with plain pitch and uv install

[CooperBigFoot]

docs(readme): rewrite top-level README for hydrologists with plain pitch and uv install

What changed

Rewrote the top-level README around the canonical plain-language watershed pitch, changed install guidance to uv add pyshed with a small pip install pyshed fallback, deleted numeric performance figures in favor of qualitative first-open-is-slower guidance, removed "and friends", stripped engineering jargon and em dashes, and reworded the PyO3 repository-layout row.

Why

The README's audience is hydrologists; it carried unexplained storage-engine jargon, unbacked performance numbers, and a pip-first install that the docs campaign standardizes on uv.

Verification

1. /Users/nicolaslazaro/Desktop/work/shed/crates/python/.venv/bin/mkdocs build --strict -f mkdocs.yml -d .mkdocs-site-check && rm -rf .mkdocs-site-check: passed.
2. ! grep -q '—' README.md: passed.
3. ! grep -qi 'and friends' README.md: passed.
4. ! grep -q 'HDX' README.md: passed.
5. ! grep -Eiq '[0-9]+([.,][0-9]+)?[[:space:]]*(ms|millisecond|second|minute)' README.md: passed.
6. grep -q 'uv add pyshed' README.md: passed.
7. grep -q 'pip install pyshed' README.md: passed.
8. grep -q 'the whole upstream area that drains to it' README.md: passed.
9. ! grep -Eiq 'row[- ]group|range read|rayon|pyo3|projjson|hilbert|xdg|object[- ]stor' README.md: passed.
10. ! grep -q 'COG' README.md: passed.
11. ! grep -Eiq "isn'?t just|not just|not merely|not only|it'?s not|it is not" README.md: passed.
12. grep -qi 'watershed' README.md && ! grep -qi 'drainage basin' README.md: passed.
13. test "$(git diff --name-only)" = "README.md": passed before commit.
14. test "$(git diff --name-only HEAD~1 HEAD)" = "README.md": passed after commit.