Analytical docs in STE, and a gate that makes the rule durable (#291)

[jdatcmd]

Completes the documentation half of #291, and then does the part that makes it
last: it checks the rule.

Five-major matrix: ALL VERSIONS PASSED, docs_style green on all five.

Converted

file	long sentences before	after
limitations.md	51	0
benchmarks.md	22	0
testing.md	16	0
ARCHITECTURE.md	15	0
features.md	17	0
index.md	1	0

features.md and index.md were in neither scope list. A project-wide rule has
to cover them, so they are in.

Structural rules only for these, as agreed: they argue rather than instruct, and
the restricted vocabulary would blunt them. Two data-flow paragraphs in
ARCHITECTURE.md became numbered steps instead. They were diagrams written as
prose, and splitting them into sentences would have kept the length rule and lost
the shape.

The durable part

test/docs_style.sh runs in the matrix. A document that drifts goes red.

A rule that nothing checks is a rule the next writer does not know about, and this
project has paid for that shape once already: an empty REGRESS made
make installcheck report success while running nothing.

Four rules over docs/*.md and README.md: no em or en dash, no double hyphen
used as a dash in prose, 25 words to a sentence, no phrase from an idiom list.

Scope is a decision, not an oversight, and the suite says so:

• CHANGELOG.md is a record of what happened when it happened. Rewriting landed
  entries would edit history, so it is checked for dash characters only.
• design/ and the code comments are out of scope. Both explain why a thing is
  the way it is, and that reasoning is worth more than uniformity.

Proved by removal, each rule separately

33-word sentence added   -> FAIL ... 1 long
"falls back to" added    -> FAIL ... 1 idiom
em dash added            -> FAIL ... 1 em/en dash
em dash in CHANGELOG     -> FAIL CHANGELOG.md carries no em or en dash

Plus an anti-vacuity control: the suite asserts it examined at least ten
documents, so a broken glob cannot pass as a clean run.

My first attempt at the length proof used a 24-word sentence and did not fire.
That was my error and not the checker's, and it is the reason the proof above
states the word count.

What is not claimed

Full ASD-STE100 compliance. It is defined against the licensed ASD Dictionary
of roughly 900 approved words, which this project does not have. The
approved-vocabulary rule is not enforced and is not claimed, in the suite, in the
checker, or in docs/testing.md. An unverifiable claim of compliance would be
worse than an honest partial one.

One thing worth reporting

The gate found three violations in the section of docs/testing.md that
documents the gate. That is the most direct evidence I can offer that it works.