check-agent-record stores a measurement of model-matrix.md, so re-anchoring a citation forces an edit to the checker and its test

Row: -

Owed by the model matrix. .agents/specs/upstream-pin-advance-e126687.md lists it
under ## Owed.

Found by the fresh review of #2975 (wave MATRIX), which repaired #2819's stale
matrix citations. Not fixed there: that change is a records-only edit across a
keyed matrix, and removing the coupling is a change to a gate's semantics, which
AGENTS.md §"Changing the rules or a checker" says needs its own spec and a
red-before test.

The defect

check_model_invariants in scripts/check-agent-record.py stores a measurement
of .agents/model-matrix.md inside scripts/check-agent-record.py:

    expected = {
        "rows": 324,
        "memberships": 373,
        "architectures": 356,
        "targets": 309,
        "modules": 245,
    }

targets and modules are derived from the matrix's Upstream cell, so they move
whenever a citation is re-anchored — not only when a row is added, removed,
merged or re-aliased.

AGENTS.md §"Records" states the rule this breaks in two sentences:

Never store a measurement of one file inside another file. A number that
changes after each edit couples every pull request to lines that it does not
own.

and names the remedy:

A gate often creates the lock. If a checker requires every change to edit one
shared file, the checker is defective. Move the obligation to a per-row
surface. Do not delete the obligation.

#2975 is its own proof

#2975 added no row, removed none, merged none, re-aliased none. rows,
memberships and architectures are all unchanged. It re-anchored twenty
Upstream cells onto symbols that still exist at the e126687a9a pin, and that
alone forced edits to two files it does not otherwise own:

• scripts/check-agent-record.py — targets 310 → 309, modules 261 → 245
• tests/scripts/test_agent_record.py — the fixture that pins those constants

So a matrix repair, the most per-row edit this repository has, writes a shared
checker and a shared test. N concurrent matrix pull requests all write those two
files, which is the lock shape AGENTS.md §"Records" describes and the shape that
made the retired .agents/issue-index.md the repository's dominant conflict
source (#883, #2290).

What is owed

1. Derive the invariant at read time instead of storing it, or move the
   obligation to a per-row surface (one file per row, read with a glob), as
   AGENTS.md §"Records" prescribes. Do NOT delete the invariant: it catches a
   silently dropped or duplicated row, which is the failure it was written for.
2. Keep whatever replaces it insensitive to a citation re-anchor, so the only
   edits that move it are the inventory changes it means to gate.
3. Red-before evidence for the replacement: a deleted row must still red, and a
   re-anchored citation must NOT.

What this issue is NOT

It is not a claim that check_model_invariants is wrong about today's tree. The
constants in #2975 are correct, were re-derived from the matrix, and were proven
red against the old matrix. The defect is the coupling, not the value.

FOLLOWING_AGENTS_PROTOCOL

Start with AGENTS.md sections “Records” and “Changing the rules or a checker,” then inspect check_model_invariants in scripts/check-agent-record.py and its fixture in tests/scripts/test_agent_record.py. Run the existing checker and test before changing them; done means a deleted row is still red while a re-anchored citation no longer requires shared checker or test edits.