Add numeric type cases to CustomTypeSerializer

[SirCotare]

Ported from #61.

CustomTypeSerializer.serialize() was missing explicit handling for numeric types (Byte, Short, Integer, Long, Float, Double, BigInteger, BigDecimal, ScaledBigDecimal). These fell through to the generic writeRawValue() branch, causing numeric-wrapped CustomType<?> values to be serialized as JSON strings instead of JSON numbers.

This port adds the nine missing case branches, each calling writeNumber() with the appropriately typed value. The implementation is adapted from the v1 branch fix (PR #61) to the Jackson 3.x API used in main (tools.jackson.*, ValueSerializer, SerializationContext, no throws IOException). No case null branch was added — the value is @NonNull per jspecify and the null case is not applicable in this branch.

Review notes

This PR ports an existing change into this project. Please focus on:

• Completeness — are all relevant changes from the source correctly reflected here?
• Integration — does the ported code fit into this project's existing patterns, conventions, and call sites?
• Adaptation — were any necessary adaptations (naming, imports, framework differences) made correctly?