docs: plain-prose pass over README

[lesnik512]

Prose pass over README.md, the only user-facing Markdown in this repo (no mkdocs site; docs/ holds ADRs and agent notes, which are out of scope). Code blocks, inline code, link targets and the pyproject description are unchanged.

What changed

• Em dashes in prose: 25 -> 0. Replaced with commas, colons, semicolons, parentheses or a sentence split.
• Bold run-in labels: 8 -> 0 (AsgiFastStream note, "Tuning", "Must be outermost", "Fix", "These raise...", "Still unguarded", and the two bold-led bullets under Limitations > Other). Rewritten as plain sentences.
• Numbered "How it works" list with bold labels turned into ### subsections (Message dispatch, Concurrency control, Offset committing, Rebalance handling, Shutdown).
• Decorative bold inside sentences removed (not, outer, before, after, first): 6 instances.
• Headings to sentence case: Quick start, Core concepts, API reference, How it works.
• Smaller fixes: dropped a duplicated "other policies pass through untouched" clause in Quick start, "simply passes through" -> "passes through", "restore unbounded buffering" -> "allow unbounded buffering" (version-history wording).

Anchors

• The Core concepts link "Rebalance handling" pointed at #how-it-works; it now points at the new #rebalance-handling subsection.
• "See Quick Start for usage examples" is now a link to #quick-start.
• No other file in the repo links to the renamed headings.

Left on purpose

• Three **not honoured** cells in the control-signals table: the following paragraph refers to the rows "marked not honoured", so the marking carries meaning.
• The emoji footer headings (PyPI, License) match the org-wide README footer.

Verification

• just lint-ci: passes.
• Tests: 209 passed against a throwaway Redpanda container on port 39192 (KAFKA_BOOTSTRAP_SERVERS=localhost:39192 uv run --no-sync pytest), container removed afterwards.