feat: add sensitive struct

[ccharly]

Add a new struct sensitive that can used with object or type. Any value for this field will get redacted automatically if any validation/errors happen.

This also apply if a sibling field validation is failing, so there's no way to make the value escape out of the actual payload being validated.

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Note

Medium Risk
Changes failure payloads and messages for schemas using sensitive fields—intentional for security but may break consumers that logged full values; validation semantics are unchanged.

Overview
Adds sensitive(), a struct wrapper that keeps validation behavior the same but replaces secret material in failures with *** (SENSITIVE_REDACTED) in value, default message, and branch (including nested entry failures and refiner names without leaking the raw value).

object() and type() now detect sensitive-marked fields and wrap sibling entry structs so when another field fails, parent objects in branch are shallow-copied with sensitive keys redacted; this uses context.branch so coercion does not break reference matching.

The Struct constructor copies Symbol-keyed properties from spread props so brands (e.g. after optional) still propagate. Public exports include sensitive and SENSITIVE_REDACTED; changelog and validation tests cover siblings, coercion, exactOptional, outer sensitive(object(...)), and non-plain objects.

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[ccharly]

This is necessary to forward the brand (marker) inside structs when we are re-constructing them.

Type-wise, that does not change anything, those are just plain symbols that are used as markers for us to detect later if a re-constructed struct was banded as sensitive (we could use this same logic for other kind of decorators in the future too).

[hmalik88]

LGTM, but want @Mrtenz to take a final look 🙇‍♂️