test(gate-16): the fixture #364 shipped without

[rubenvdlinde]

#364 (merged as 765d081) changed a gate verdict fleet-wide and shipped with no
fixture behind it. This is the fixture, driven through the real bin/hydra-gates
over a purpose-built two-commit repository, and observed RED before the change
and GREEN after:

tree	result
81c8c97 (pre-#364)	3 failed / 9 passed — arm 3: ".github#361 is LIVE: gate-16 printed PASS on a --full run that computed no diff, over a tree whose two untagged methods the positive control just named"
765d0817 (post-#364)	0 failed / 12 passed

Both of #364's load-bearing claims were re-derived, not taken on trust

Each by patching the runner and re-running this same fixture:

counterfactual	observed
_skip 16 … scope instead of na	[gate-16] spec-coverage: FAIL — internal error: _skip called with reason category 'scope' — a fleet-wide false RED, exactly as #364 states. The suite now asserts against it by name.
a full sweep on --full (the gate-19 else-branch shape)	[gate-16] FAIL — 2 changed method(s) missing @spec, naming LegacyDebtController, which no author touched. ADR-020 violated.

⚠️ One refinement to the second claim, for the record. A literal copy of
gate-19's else-branch — merely dropping HYDRA_GATE_BASE_REF= — would not have
swept at all: check_spec_coverage.py:508 defaults the env lookup to
origin/development, so it returns # count=0 and changes nothing. The
false-RED danger is real, but it arrives one line later, with the
check_visual_coverage.py or None precedent. Nothing in #364 is wrong; the
mechanism is one step removed from where the PR body places it.

The arms

All through bin/hydra-gates, one tree, only the scope input changed:

positive control  --mode report NAMES LegacyDebtController, uncovered_count=2
arm 1  diff, docs only          PASS      (ADR-020: inherited debt is not mine)
arm 2  diff ADDS untagged work  FAIL, and NAMES NewWorkController.php
arm 3  --full, arm 1's tree     NOT APPLICABLE, reason names the ABSENCE of a
                                diff, no legacy finding in the log at all,
                                gate-16 listed in the coverage summary's NA block

Arms 1 and 2 are what make arm 3 readable: without them a NOT APPLICABLE is
ambiguous between "gate-16 is honest about an empty scope" and "this fixture
contains nothing to find".

Arm 3 asserts the absence of LegacyDebtController from the gate's log, not
just the absence of a FAIL verdict — so a full sweep cannot creep back in wearing
a NOT APPLICABLE.

Coverage ratchet

gate-16 moves from UNCOVERED.md (needs-diff, whose stated reason — "it needs a
purpose-built two-commit history, not a fixture tree" — was accurate and is now
satisfied) to COVERED-ELSEWHERE.md. 12 of 64 fixtured → 13 of 64.

Verified green on origin/main at 765d081: test_gate16_spec_coverage_scope.sh
12/12, test_gate_acceptance_matrix.sh 4/4, test_gate_crashed_checker_is_not_a_finding.sh
ALL PASS, ShellCheck clean (koalaman/shellcheck:stable against the repo's own
.shellcheckrc).