docs: tidy readme prose

[lesnik512]

Summary

Readme prose fixes. No code or metadata changes.

Changes

• readme.md:28 said ruff and ty do linting and formatting; ty is a type checker. Now "Linting and formatting with ruff, type checking with ty".
• Headings: "Key Features" is sentence case; "### Description" (directly under the # title) and "### After git clone run" became ##.
• readme.md:14-16: the em-dash aside became a colon list plus a separate sentence.
• readme.md:46-49: "Nothing shows them composed under load-bearing decisions ..." rewritten as two plain sentences that keep the same three facts (transaction spanning two writes, unique constraint hit by concurrent requests, count that must not cost a row per event).
• The em dash in the shared "Part of modern-python" footer is left as is; the same line appears in 24 org readmes.
• Every path in the pattern table was checked and still exists.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging

Tests ran against a throwaway postgres:17 container: 110 passed at 100% coverage, plus 4 migration tests.