docs: align use-case claims and plainify prose

[cardmagic]

Two documentation commits, no code changes.

1. docs: align use-case claims with the JS port — brings the Ruby
   use-case wording in line with the JS port, matching
   docs: align use-case claims with the Ruby gem solid-objects-js#30.
2. docs: use plainer prose in README and guides — removes the
   rhetorical constructions that read as machine-written.

What the prose pass changed

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 gem and it works" → "That needs less code than this gem, and it is sufficient for that case"
Personification	"The lock ... has no say over which push arrives last" → "The lock sets the write order, but it does not set the order in which the pushes arrive"
Aphorism	"Sequential does not mean once" → "Ordered execution does not prevent repeated execution"

Validation

Markdown only. No Ruby source, signature, migration, or test file is
touched, so rake behavior is unchanged. Every guarantee, benchmark
number, policy name, and code sample is preserved verbatim.