Adds the metadata sanitization layer per #REQ-AUD-5:
SIGCM2.Infrastructure/Audit/JsonSanitizer.cs (static class):
- Sanitize(object?, IReadOnlyCollection<string>) -> string?
- Serializes via System.Text.Json + JsonNode recursive traversal.
- Strips blacklisted keys at every nesting level (objects + arrays).
- Case-insensitive match (ToLowerInvariant on both sides).
- Null input -> null output (never throws).
- Output is always valid JSON (ISJSON=1 compatible — satisfies AuditEvent CHECK).
SIGCM2.Application/Audit/AuditOptions.cs:
- Documented the IConfiguration array-binding quirk: config is ADDITIVE
(append at higher indices), not REPLACE. Intentional for security — defaults
like 'password'/'token'/'cvv' must not be silently dropped.
SIGCM2.Infrastructure/DependencyInjection.cs:
- services.Configure<AuditOptions>(configuration.GetSection(AuditOptions.SectionName))
wired in AddInfrastructure().
Tests (Strict TDD, RED -> GREEN):
- JsonSanitizerTests (10): null/empty-blacklist/flat/nested/arrays/case-insensitive/
primitives/round-trip-valid-json/string-as-value/default-keys-effective.
- AuditOptionsBindingTests (2): defaults when section absent + additive override.
One test needed adjustment during GREEN: 'AlreadySerializedJsonString' originally
asserted against an encoding-specific literal; rewrote to use JsonDocument
round-trip (validates behavior without coupling to encoder quirks).
Suite: 348/348 Application.Tests + 130/130 Api.Tests = 478/478 passing.
Refs: sdd/udt-010-auditoria-trazabilidad/{spec#REQ-AUD-5, design#D-5, tasks#B3}
