fix: replace crypto.BinaryLike with Buffer in KeyUtils for @types/nod…

[aharoitx]

Problem

Dependabot's @types/node 25 → 26 bump (#352, previously #346) fails CI:
all jobs run npm run build, and tsc errors with:

src/util/KeyUtils.ts(43,52): error TS2345: Argument of type 'BinaryLike' is not
assignable to parameter of type 'string | ArrayBufferView<ArrayBufferLike>'.
src/util/KeyUtils.ts(50,51): error TS2345: (same)

In @types/node 26, crypto.Hash.update() no longer accepts a plain
ArrayBuffer, which the crypto.BinaryLike type includes.

Solution

Type the two private helpers in KeyUtils (and one local variable) as Buffer
instead of crypto.BinaryLike — Buffer is what they actually receive from
their only call sites. Type-only change; the emitted JavaScript is identical.

Verification

• npm run build passes with both @types/node 25.9.3 (current) and 26.1.0
• npm run unit: 55/55 tests pass, identical results before/after
• format:ci and lint pass