repsel: promotion census — make "zero values promoted" a visible, gated state instead of an invisible one

[proggeramlug]

--opt-report (#6952) can tell you, for one build, which values got an unboxed
representation and which did not. What it cannot do is tell you that the answer
changed — or that it has been zero all along.

#7034 established the state of things by hand-instrumenting the compiler:

• Ptr<Shape> promotes nothing at all on benchmarks/app-patterns/kernels/batch.ts,
  the object/property-heavy program that representation exists for.
  PERRY_PTR_SHAPE_LOCALS=0 versus the default produced a byte-identical
  binary (4,294,904 bytes, __text 3,804,424).
• Across benchmarks/suite, only 3 files promote a shape local, and each
  promotes exactly one.

Nobody knew any of that until somebody went looking. That is the failure to
make impossible to repeat.

Ask

A per-workload, per-representation census recorded as a ratcheted baseline,
so a drop turns a build red.

Per representation, not aggregated: the interesting signal is Ptr<Shape> = 0
while canonical Str ≠ 0. One total hides exactly that.

The hard part

CLAUDE.md, "★ Four ways a gate can be unable to fail", case 4 — the gate runs
but its subject never did. A census that faithfully reports Ptr<Shape>: 0
and exits green is worth nothing: that is the state the project is in
today. So the census needs, beyond floors:

• floors that can actually be violated (the honest floor for Ptr<Shape> on
  real code is zero, and a zero floor can never fail);
• a liveness assertion on the instrument itself — a fixture that definitely
  promotes, which fails the census if its count reads zero, so that "zero
  because nothing promoted" is distinguishable from "zero because nobody
  increments the counter";
• verification by sabotage in both directions.

Related

• repsel row 1: scoping Ptr<Shape> beyond locals — measured, promotion is 0 on the motivating workload; build --opt-report (#6952) first #7034 — the scoping analysis with the original numbers.
• feat(cli): --opt-report — surface which values could not be statically typed, why, and whether the developer can fix it #6952 — --opt-report, the data source.
• opt-report v2: LocalId → Span side-table so --opt-report can print file:line and a source snippet #7036 — source spans for --opt-report. Separate, additive.