Give a skipped readback criterion its own state, and count criteria instead of declaring 12

[TennyZhuang]

The readback criteria set is the last instrument in this job still running on two
states.

When build_assets_legacy is absent, two criteria fall into an else that prints
(no build_assets_legacy; nothing to compare the copy against) — and the run goes
green. A criterion silently not executing is exactly the shape exit 2 was
introduced elsewhere to prevent; this is the one place it had not been applied.

Latent today, permanent later. The table still exists, so the branch is not taken.
It becomes permanent the moment task #114 drops the table — after which the deploy log
prints a reassuring sentence forever while checking nothing.

Change

check()	increments evaluated
skip()	new — prints SKIP <criterion>: <reason>, increments skipped
else	reports both criteria as SKIP instead of one parenthetical
end	evaluated N criteria, M skipped, … failed; warns when M > 0

Skips are loud, not red. A skip can be legitimate, and failing the deploy on one
would hold production hostage to a comparison source we intend to delete. What it must
not be is quiet — a permanently skipped criterion is itself the event worth seeing, and
it now says so on every deploy instead of waiting for someone to remember.

Counted, not declared. This gate has been described in writing as "12 criteria"; a
number kept in prose stays at 12 while the code drifts. Same reason the workflow lint
reports N of M YAML files with both figures counted.

Why this is not bundled into #114

That was the earlier plan and I now think it was wrong. #114's deletion is irreversible,
needs its own explicit authorisation, and may be executed under time pressure — a fix
riding along with it is a fix riding the least careful moment.

Done separately, while the table still exists, it is also verifiable now:

legacy present  -> evaluated=2 skipped=0 fail=0
legacy absent   -> evaluated=0 skipped=2 fail=0   <- previously printed green, silently
real mismatch   -> fail=1                          <- still fails

Once the else branch is the only reachable one, that comparison is no longer possible.

Verification

• three branches exercised as above
• workflow lint: exit 0
• deploy job parses, environment: gate intact, 16 steps
• full worker suite: 403 passed, 35 files

Credit

The design — giving the skipped state a name — is @sentinel's. Counting the criteria
rather than asserting the total is @gogo's.