sl0ppty: layouts are `*.layout.kdl`, and each loader says which it got

`config/layout.example.kdl` becomes `config/example.layout.kdl`, and the
convention for the files people write is `project.layout.kdl`.

Not a new extension, and the reason matters: the syntax genuinely *is* the same
hand-rolled KDL subset for both documents -- one parser reads them -- so calling
layouts something other than KDL would trade one wrong signal for another and
lose every editor that highlights it. The confusion is about *schema*, so the
schema goes in the stem: the extension says how to read the file, `.layout` says
what is in it. `docker-compose.yml` and `tsconfig.json` settled this convention
long ago.

**But a filename cannot enforce anything, and that turned out to be the real
bug.** `sl0ppty --check` on a layout file printed `ok` and reported "bindings 53"
-- it had read a document containing nothing it understood and said so was fine.
A config that `include`d a layout linted clean for the same reason. So did
`rouded true`. The loader reads a document by *asking it for the keys it knows*,
which is fast and clean and never notices what it was given.

So it notices now. `KNOWN_TOP` is every node the loader reads, and anything else
at the top level is a complaint with a file and line:

    s.kdl:2: unknown setting: rouded
    example.layout.kdl:14: this is a layout, not a config: `sl0ppty --layout` reads those
    sl0ppty: config.kdl: this is a config, not a layout: `gap` is a setting

The `layout` node gets its own message because it is the mistake people actually
make, and the reverse direction is the same list read backwards -- the layout
loader asks `config_is_setting()` rather than keeping a guess of its own, so the
two answers cannot disagree about which file is which. A layout whose own tabs
are misspelled still gets "layout declares no tabs", which is a different
mistake.

D2 records it, since it is the decision that two schemas share one syntax.

Verified: `tests/test_config.py` gained thirteen checks -- the unknown setting
with its line, both wrong-schema messages, the misspelled-tabs case keeping its
own, the shipped config linting clean and the shipped layout loading as one, and
a drift guard that greps `kdl_child(root, ...)` out of the loader and fails if a
setting is missing from `KNOWN_TOP` (verified by removing `gap_aspect` and
watching it fail). Full suite green; my own config and the reference config both
still lint clean, which is the thing this change could most easily have broken.

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