Fix (Value): Union encode selects member matching the unencoded value (#1706)

[cpruijsen]

Value.Encode and EncodeUnsafe now try union members that accept the unencoded value first (src/value/codec/from_union.ts).
For Union([Codec, Null]), a null input returns null without a call to the codec's encoder.
Previously, a strict codec could throw, while a JSON codec could return the string "null".

Each member attempt receives a clone and must produce output that passes that member's check.
The fallback handles decoded Date and object values that do not match a member before encoding.
The fallback skips members already attempted in the first pass.

Union([Codec(String), Number]) now encodes numeric 5 as 5 when the codec decodes to numbers.
Previously, that codec could return "5" first.
The extra input checks can increase validation cost.

The strict Date and JSON null regressions in test/typebox/runtime/value/codec/union.ts fail with the merge-base source and pass with this change.

Fixes #1706.