feat(cli): colitur check and colitur new-overlay

Writing a local calendar had no feedback loop. An overlay is applied, not
validated -- that stays true, and the five test layers still cannot vouch for
a user's file -- but before this the only way to learn whether yours did what
you meant was to generate a year of output and grep for your own slug, and the
only way to see that a directive matched nothing was to notice a warning
scroll past among 365 lines.

`check` loads each overlay, applies it to the real shipped calendar, and
reports the directive counts, the slug each one targets, and any directive
that found no target. It exits 2 when a file fails to load or a directive
matched nothing, so it composes into a Makefile or a pre-commit hook rather
than merely being readable. It is applied to the SHIPPED calendar and not to
an empty layer on purpose: against an empty one every Suppress would fail
trivially and the check would be worthless.

It answers three narrow questions -- does the file parse, does every directive
find its target, what does the merged result contain. It does not validate a
calendar against the rubrics and cannot, and both the help text and the man
page say so rather than letting the name imply more than it does.

`new-overlay` prints a starter to stdout for redirection, rather than writing
a file where it likes. Every value in it is a placeholder that will appear in
`day` output if left unedited, so a half-finished overlay is visible rather
than silently inert, and it documents the three date shapes and the legal
values for each closed field inline.

load_ef_layer now returns its diagnostics instead of printing them: day and
readings still want them on stderr beside a year of output, while check wants
them on stdout, attributed to the overlay that produced them, and counted.
Printing at the source made the second impossible.

The cram test round-trips new-overlay through check rather than pinning the
template line by line -- editing its prose should not fail a test, but a
syntax error in it still must.

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