ir: IsSubtype walks the hierarchy afresh per call, so its callers go quadratic #850

Problem

ir.IsSubtype walks the target's supertypes from scratch on every call, so code that asks it once per route of a discriminator does work proportional to routes × hierarchy depth. pass/validate asks it for every mapping target of every model discriminator (checkModelDiscriminator). A generated document with N models, each mapping to the bottom of an N-deep allOf chain, validates in time quadratic in its size:

# N chained models, then N discriminated bases that each map to the deepest one
D = K = N
print("openapi: 3.1.0\ninfo: {title: t, version: '1'}\npaths: {}\ncomponents:\n  schemas:")
print("    C0: {type: object, properties: {kind: {type: string}}}")
for i in range(1, D + 1):
    print(f"    C{i}: {{allOf: [{{$ref: '#/components/schemas/C{i-1}'}}], type: object}}")
for k in range(K):
    print(f"    M{k}: {{type: object, discriminator: {{propertyName: kind, mapping: {{x: '#/components/schemas/C{D}'}}}}}}")

Measured on main at a2530c37. The lowering is linear; the pass roughly triples with each doubling.

#808 adds a second caller. The OpenAPI compiler settles a discriminator beside a kept union by asking ir.IsSubtype of each route, union branches included. A document whose N degraded models each name the bottom of the chain as a branch shows the same curve inside the compile.

Expected

Asking whether many types are subtypes of a base costs time linear in the hierarchy plus the number of questions. Two shapes would do it:

• A per-document index built once (ir.SubtypeIndex or similar) that answers IsSubtype from memoized supertype closures, or from DFS intervals where each type has one supertype. A DAG with Implements needs the general form.
• Callers group their questions by base and walk down once from the base over a reverse Base index.

Either one must answer exactly what IsSubtype answers today, cycles and alias reads included. That is easiest to guarantee by making IsSubtype itself the index's thin wrapper rather than a second implementation.

Acceptance

• Validating the generated document grows linearly with N, measured as work counted rather than wall-clock.
• Both callers go through the same implementation.