docs(pyshed): slim README to quickstart, move tuning knobs to API.md

[CooperBigFoot]

docs(pyshed): slim README to quickstart, move tuning knobs to API.md

What changed

• Rewrote crates/python/README.md as a short hydrologist-facing quickstart.
• Added ### Tuning Knobs to crates/python/API.md with guidance for the four constructor knobs.

Why

• Keep the PyPI page focused on install, hosted data, local data, and engine reuse.
• Move tuning guidance into the developer API reference beside the constructor table.

Verification

• mkdocs build --strict -f mkdocs.yml -d .mkdocs-site-check && rm -rf .mkdocs-site-check: passed.
• ! grep -nE 'parquet_cache|snap_radius|repair_geometry' crates/python/README.md: passed.
• ! grep -n '—' crates/python/README.md: passed.
• ! grep -n 'HDX' crates/python/README.md: passed.
• ! grep -nEi '[0-9][0-9,.]*[[:space:]]*(ms|msec|millisecond|second|minute)' crates/python/README.md: passed.
• ! grep -nEi ", not (a|an|the) |isn'?t just|not just|it'?s not|mechanism, not" crates/python/README.md: passed.
• Required README strings gate: passed with FACTS-OK.
• grep -q '^### Tuning Knobs$' crates/python/API.md && echo SECTION-OK: passed with SECTION-OK.
• All four knobs documented in the new API section: passed with KNOBS-OK.
• ! sed -n '/^### Tuning Knobs$/,/^### Methods$/p' crates/python/API.md | grep -q '—': passed.
• API.md insertion-only diff gate: passed with INSERT-ONLY-OK.
• Exactly two tracked files changed before commit: passed with SCOPE-OK.