docs: use plainer prose in README and guides

[cardmagic]

Prose-only pass over the README and docs/. No source, type, test, or
fixture file is touched.

Pattern	Before → After
Cleft "X is what Y"	"The combination is what costs" → "These three must agree with each other"
"X, not Y" closer	"The argument for an actor is scope, not discipline" → "An actor helps when the work goes outside the scope of a lock"
Concessive flourish	"That is less code than this library and it works" → "That needs less code than this library, and it is sufficient for that case"
Cliché	"The database is the source of truth" → "The database holds the authoritative state"
Marketing aside	"more than a counter-shaped happy path" → "more than a simple counter"
Metaphor	"Two companions complete the local-first story" → "Two more modules support local-first applications"
Imperative chant	"Choose celld to ... Choose Solid Objects to ..." → one comparative sentence

Also removed the last em dash in CHANGELOG.md and reworded the parity
ledger's "the wire contract is what both sides guarantee".

Validation

• node scripts/check-documentation.mjs — exit 0
• node scripts/check-parameter-style.mjs — exit 0
• prettier --check README.md CHANGELOG.md docs/*.md — clean

Kept intact: the delivery-boundaries list, the parity statuses, every
benchmark figure, and the Shuffle Up and Play evidence table.

[greptile-apps]

Greptile Summary

This PR rewrites README, changelog, API, authorization, benchmark, correctness, and parity documentation in plainer language without changing documented behavior.

• Simplifies prose and removes metaphors, clichés, and marketing language.
• Preserves API contracts, correctness boundaries, benchmark figures, compatibility claims, links, tables, and code examples.

Confidence Score: 5/5

The documentation-only changes appear safe to merge.

The revised prose preserves the documented runtime contracts, security boundaries, compatibility requirements, benchmark data, Markdown structure, and references, with no actionable defect identified.

Important Files Changed

Filename	Overview
README.md	Rewords product guidance and runtime explanations while preserving the documented guarantees and limitations.
CHANGELOG.md	Replaces an em dash with sentence punctuation without changing the release note.
docs/api.md	Clarifies that TypeScript declarations define exact signatures without altering API guidance.
docs/authorization.md	Simplifies authorization prose while retaining the deny-by-default security boundary.
docs/benchmarks.md	Rephrases benchmark limitations without changing figures or methodology.
docs/correctness.md	Simplifies delivery and wake-up descriptions while preserving at-least-once and authoritative-state guarantees.
docs/parity.md	Rewords parity and wire-contract explanations without changing compatibility statuses or payload requirements.

_{Reviews (1): Last reviewed commit: "docs: use plainer prose in README and gu..." | Re-trigger Greptile}