TS => MJS: basen/base128 - #1463

Approving. Small, clean migration — proper rename, CHANGELOG entry present, two importers updated.

Signatures are preserved exactly, checked with a strict Equal<A,B> against the pre-migration types rather than by reading:

typeof encode === (uint: bigint) => Vec                    ✓
typeof decode === (v: Vec) => readonly[bigint, Vec]        ✓

and they're genuinely enforced — encode(5), encode(5n) assigned to string, and decode(5n) each produce the right specific error, so nothing collapsed to any.

npx tsc clean, npm test → pass: 2354, fail: 0, total: 2354, CI 19/19.

No typedefs in this module, so the new _ convention has nothing to apply to here — worth noting only because this is the first migration since #1462 landed it.

One small thing done well: keeping @param / @returns rather than collapsing to a single @type preserves the parameter names and descriptions the original TypeScript JSDoc carried. A @type annotation would have type-checked identically but dropped "The bigint to encode" and "The MSB Base128 vector to decode" from the emitted declarations.