docs: a review pass, fifteen claims the code contradicts

[vyncint]

Each was checked against the code or by running it before it changed;
one more item is a style tidy, marked as such.

README:

• Screen::parse reads the snapshot format, not an insta .snap: it
  rejects the --- header on line 1. The body below it parses, and the
  termlens command reads a whole .snap because it strips the header.
• The skill has five recipes, not four.

SKILL.md:

• §7's mask row said mask_matching "spans a wrap the way find_all
  does". A value split by a soft wrap is two rows and stays unmasked
  (measured: 10 columns, SECRET-VALUE-42); a needle with \n is what
  spans rows. Rule 12 already says so for contains.
• Rule 10 said format!("{:?}") is the same as Display. It wraps the
  text in Screen(…), which Screen::parse, diff and render refuse.
• §6 said every error's Display ends with the screen. Only the wait
  errors (timeout, EOF, emulator) and a failed write carry one.

SECURITY.md: "bot-authored commits are rejected by CI" is true except for
Dependabot, which CONTRIBUTING's AI tooling policy exempts from the
identity rule only; the provenance line now says so.

DESIGN.md:

• The writer queue is unbounded (mpsc::channel), capped on reply bytes;
  one paragraph said bounded, the next unbounded.
• The user-facing Windows list is in docs/LIMITATIONS.md, not the README.
• CSI 18 t is answered, and 14 t/16 t once a cell size is declared,
  so "the non-pixel CSI t reports" were never all unanswered; the same
  correction in LIMITATIONS.md names 11 t and 19 t instead.
• The compatibility corpus starts at 0.10.1, not "each published release".

emit's steps: an argument beginning with -- must be a step, so emit --foo exits 2 rather than printing --foo; TEXT and --text say so.

Rustdoc:

• Unsupported's summary was Clipboard's first sentence, so docs.rs
  described the type as clipboard data; the sentence is back on
  Clipboard, which had no summary.
• assert_screen_snapshot! repeated the Display-versus-Debug reason that
  docs(skill): rule 9 gives the true reason, and §6 reads the Debug form too #559 corrected in the skill.
• wait_frame pointed at a README Windows note that is not in the README.
• Screen's Debug impl said "exactly like Display"; it adds Screen(…).
• Style, not a contradiction: mask_matching carried the crate's only
  # Examples heading; the rest of the crate's doctests have none.

Found by a documentation sweep; the same sweep found the relative
current_dir bug and the inspect help errors fixed in #561.