fix(compilers/openapi): map sized integer formats to primitives

[fuad-daoud]

Summary

formatTable mapped integer/int32, integer/int64, number/float, number/double, number/decimal and the string formats, and nothing else, so {type: integer, format: int8} lowered to an anonymous Scalar over t/prim/integer with Encoding.Name: int8 even though ir.PrimKind has PrimInt8.

This adds the seven entries naming a width of their base type — integer/int8, integer/int16, integer/uint8, integer/uint16, integer/uint32, integer/uint64, number/decimal128 — at their grouped positions, so those positions now resolve to the shared t/prim/<kind> primitive: no anonymous node, no Encoding, no diagnostic, and formatHome still reports the keyword homed through the primitive kind. formatTable's doc comment records the rule.

Deliberately unchanged: the literal float32/float64 spellings (float/double already map — the table maps OpenAPI format spellings, not TypeSpec type names), float16/uint128 (no such PrimKind), byte's own hoister, and the fallback that hoists an unlisted format as a Scalar over baseForType — including {type: string, format: int64} (#637).

The plan shipped with this fix is at docs/plans/636-sized-integer-primitives.md.

Test plan

• go test ./compilers/openapi/internal/schema/ -run TestScalar -count=1 — green. The new TestScalar_NumericFormatsMapToPrimitive covers the seven new pairs, the int32/int64 and float/double/decimal controls, and two fallback boundary rows (number/float16, string/int64); a mapped row asserts the property target is ir.PrimTypeID(kind), the node is an *ir.Primitive of that kind, and no t/anon/... node exists at the property pointer.
• Mutation probe: deleting the integer/uint32 entry reddens the uint32 subtest (t/anon/components/schemas/Holder/properties/p4 instead of t/prim/uint32); restoring it is green.
• go test ./compilers/openapi -run TestConformance -count=1 — green, no golden changed.
• make gate — exit 0 (fmt, vet, lint, nolint sweeps, build, coverage-count, coverage 100%, fuzz, bench-smoke).