test(activity): grade the DIVE-3419 PARTIAL banner at the presenter seam (DIVE-3421)

[5dive-bot]

The gap

DIVE-3419 (#648) gave activity_collect a reporting contract: .partial names every level it
could not read, and cmd_activity prints a ⚠ PARTIAL — N level(s) could not be read banner above
the counts, so a short trail can never be mistaken for a quiet one.

tests/usage_middle_wildcard_unit.sh (23 arms) grades that at the collector seam — the JSON
field, in python, from a fixture tree. It cannot see the presenter. The banner is bash+jq and
cmd_activity calls require_root, so it was undrivable from the seat that wrote it and shipped
with no arm at all. A .partial that is populated correctly and never rendered is the same
silence DIVE-3419 exists to remove, and a jq typo in the banner would have been caught by nothing.

Named by the maker at delivery rather than left for a reader to find as an oversight (DIVE-3421).

What this adds

One file, tests/activity_presenter_partial_unit.sh, 19 arms, core tier (no # TIER: marker),
0.42s. It sources src/cmd_usage.sh and drives the real cmd_activity with only
activity_collect stubbed — tests/usage_presenter_coverage_unit.sh (DIVE-1937) is the precedent
and the shape. require_root is stubbed because it is the reason this seam was untested, not the
subject; stubbing it is what buys the arms any-uid coverage.

group	arms	what it holds
anchor	2	a complete read renders its specific non-zero counts and the trail itself — so every "no banner" arm below is a statement about the banner and not about a dead renderer
the banner	7	it exists; it is on stdout, not stderr; it carries the array's length; it says SHORT of the truth, not low; it names every level verbatim, each on its own indented line; and it prints before the counts it qualifies
the DIVE-3419 decision	1	a partial read still renders the whole trail and exits 0 — this reader owes a reporting contract, not the fail-closed NOT-REACHED its spend-scanning sibling owes
the idle case	2	an empty trail with an unread level still warns; 0 is what reads as "did nothing"
over-fire control	1	collector JSON with no .partial field at all: no banner, no crash, trail intact, rc=0 (grades the // [] default)
--json	3	the field travels with the counts it shortened, causes unmangled, and the banner does not leak into the stream
--task	1	the banner prints on the task-clipped view too, not only the rolling window

Two of those are load-bearing in a way worth naming. The stdout arm: a warning on the other
stream is gone the moment anyone pipes the trail it qualifies. The position arm: both line
numbers are required to exist, because an absent banner leaves an empty string, which bash reads
as 0 in an arithmetic test and would pass the ordering check vacuously.

Grade

uid 1015, unprivileged, same harness both sides.

fixed tree (this branch)                         19 passed,  0 failed   0.42s
NEGATIVE CONTROL — origin/main's cmd_usage.sh    10 passed,  9 failed

Re-run the control:

mkdir -p /tmp/prebanner
git show origin/main:src/cmd_usage.sh > /tmp/prebanner/cmd_usage.sh
USAGE_SRC_DIR=/tmp/prebanner bash tests/activity_presenter_partial_unit.sh   # must FAIL

The pre-banner cmd_activity is byte-identical apart from the banner block, so the control does not
merely error out: the anchor and trail arms stay green there and only the banner arms red. A
control that dies on line one grades nothing, and 9/19 red with the anchor green is the difference.

Two mutations on the fixed tree, each reddening only its own arms — the arms are independently
graded, not co-satisfied:

mutation	result
banner prints (.partial // [])[0:1] (first level only)	named levels + level layout red — 17/2
banner block moved below the counts (a footnote)	banner position red only — 18/1

Verdict wiring: HARNESS-RC=1 whenever an arm reds (the changed-harnesses probe's property).
shellcheck -S error — the required context — is clean; the three -S warning hits are the same
three the precedent harness carries (SC2154/SC2164 on the shared trap+cd preamble, SC2034).

Siblings unmoved on this tree: usage_presenter_coverage 26/0, usage_middle_wildcard 23/0,
usage_coverage 11/0, usage_dispatch_flag 15/0, usage_enumeration_completeness 9/0,
heartbeat_usage_heal 27/0.

Not covered, named rather than hidden

1. activity_collect's own emission of .partial is not re-graded here — that is the collector
   seam and fix(usage): the middle wildcard is a read too — usage_collect + activity_collect (DIVE-3419) #648's harness owns it. The two together close the round trip; neither alone does.
2. The collector and presenter in one process against a real unreadable dir is still unreached
   from any non-root seat, because cmd_activity calls require_root for real. Stubbing it buys
   presenter coverage at any uid; it is not evidence that the root path resolves a real agent's home.
3. No src/ change. One CHANGELOG bullet under the existing DIVE-3419 Unreleased section.

Ordering

Stacked on dive-3419-usage-middle-wildcard (#648), and based on it here. The banner this grades
does not exist on main, so this cannot land first — merged before #648 it would be 9 red arms
against code that is not there. 5dive-cli does not auto-deploy (deliberate release-cut), so
neither merge is a ship.