ci: close the preflight parity gap for ll-identity-selftest

[gertybotbot]

make ll-identity-selftest is an unconditional step in ci.yml but was absent
from CI_REQUIRED_CHECKS and from the dispatcher's fallback lane, so
make ci-preflight never executed it. A change to scripts/ll-byte-identity.sh
passed local preflight and only failed in CI -- the exact drift the parity
self-test exists to prevent. The step also sits outside every CI-PARITY-STEPS
marker block, so the parity checker could not see the omission either.

Wire it in properly rather than exempt it: the target runs
scripts/ll-identity-selftest.sh against synthetic .ll snippets, needs no
compiler build, and completes in ~0.1s, so it costs local preflight nothing.

• CI_REQUIRED_CHECKS: add the ll-identity-selftest entry
• fallback lane: add_command "make ll-identity-selftest" + 15s estimate
• corpus-floors.tsv: ci-required-checks 32 -> 33, in this same commit as the
  floor's own failure message instructs

Also document the one remaining genuine exemption. make licenses-check
(ci.yml) is likewise unconditional, outside the markers, and absent from
CI_REQUIRED_CHECKS -- but there it is the right call, because it needs
cargo-about at a pinned version, installed by the step immediately above.
Record that rationale inline following the existing leak-scan convention, so
the next person auditing this does not have to re-derive it.

The other six unconditional out-of-marker run: make steps are already
covered via make lint or an existing add_command, so the residual after
this commit is zero.

Verification: check-preflight-ci-parity.sh 33/33 and 28/28 (rc=0),
preflight-parity-selftest.sh 5/5, ll-identity-selftest.sh 6/6,
check-corpus-floors.sh 21/21, ci.yml parses, dispatcher passes bash -n.

Closes #2860

[gertybotbot]

Closing as superseded — the fix landed independently in f1f9972e0 ("fix(ci): add ll-identity-selftest to preflight parity checks", 2026-08-07), one day before this PR was opened. Both close #2860.

Found while rebasing this branch onto 4ce27c77d after it went CONFLICTING. On current main:

• add_command "make ll-identity-selftest" is already present in the dispatcher's fallback lane (line 923) — this PR's substantive change.
• The CI_REQUIRED_CHECKS table and the --ci-required flag this PR also edited have been removed entirely, along with scripts/check-preflight-ci-parity.sh and scripts/corpus-floors.tsv, by the fixture/corpus refactor (c6a642f6e). Those hunks have no target left.
• The only unlanded residue is a 15s command_timeout_floor entry. The default floor is 0, which means "use CMD_TIMEOUT" — an ample budget for a ~0.1s self-test. No behavioural gap.

Nothing here is worth carrying forward, so this needs no review time. Reopen if the timeout floor is wanted as a standalone one-liner.