fix(ci): judge discovered self-test sets in check-step-collectors (#11815)

The gate's population was keyed on one spelling -- a `scripts/`|`packages/`
path carrying a `--self-test` flag -- which is a rule about where a self-test
lives rather than what it is. A second family (`*.selftest.sh` standalone
matrices) was invisible, and lint.yml's tolerate-and-collect block over that
family scored zero targets: its collector shape was held by a review comment.

Recognition is now keyed on two self-declaring markers, neither naming a
directory: a `--self-test` flag after any repo script path, or a basename
ending `.selftest.sh`. Generalising the flag marker's path moved nothing on
this tree (31 steps with >=1 target, 2 with >=2, before and after).

Widening the markers alone was not enough, and this is the half the filing did
not have: that block names no self-test at all -- it enumerates them with
`find` and loops. So an enumerated set is recognised too, and judged PLURAL BY
CONSTRUCTION: the set can gain a member with no edit to the workflow, which is
the #10814 defect with a wider blast radius than the named case. The
recognised enumeration spellings are published in the header and the failure
text, and each is pinned.

The dynamic half drives a discovered-set block by planting stubs where the
block's own enumeration reaches them, so the block discovers them itself and
nothing is substituted into its text. Both reds are pinned (bare loop; set
routed past an existing collector), plus the empty-discovery #4690 control and
the pre-fix ablation.


Claude-Session: https://claude.ai/code/session_015ahemw8RcTgqtxrj15PEZx

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-steve