required check disagrees with decoding on key casing and repeated values #103

The required check matches keys differently from the decoder, so a required field can be reported empty although it was decoded, or pass although it decoded to an empty value. Both came up in #100 and exist on main for every field type; map fields from #100 inherit them.

type Phone struct {
    Number string `schema:"number"`
}
type S struct {
    X     string `schema:"x,required"`
    Phone Phone  `schema:"phone,required"`
}

1. Key casing

Fields match case-insensitively, required keys only case-sensitively.

DecodeValues behaves the same. directlyProvided probes src.lookup(path) and markProvidedByNestedKey probes info.requiredPrefixes[key[:off]], both exact, while the decoder resolves the field through the lowercased alias.

2. Repeated values

A scalar field takes the last of several values, but isEmpty checks value[0].

The second case lets a required field through empty. isEmpty should check the last value for the scalar kinds.

Notes

• The required check should stay allocation-free on the decode path; the existing benchmarks should stay flat.
• feat: decode map fields from dotted keys #100 changes buildRequiredPrefixes, so this is easiest after it merges.
• One test per row above, for Decode and DecodeValues.

@IslamElsayed offered to pick this up in #100.