signing: implement tenant-aware SigningContext and secure kid resolver

[bokelley]

Implement the D22 tenant-aware signing SPI and secure kid resolver shape from PR #23.

Scope:

• Implement SigningContext parameter set: AdcpUse use(), nullable tenant identity, nullable principal reference.
• Ship SigningProvider without any SigningProvider.forUse(AdcpUse) short form.
• Implement inbound verification through VerificationKeyResolver: start from inbound kid, resolve key/tenant/principal, then verify signature and JWK adcp_use.
• Treat kid as an opaque lookup key into a pre-provisioned key set.
• Never dereference attacker-controlled URLs from an inbound signed object.
• Any resolver HTTP fetch for JWKS/key refresh must use the strict SSRF-safe HTTP client.
• If verification returns no tenant/principal, receivers must treat the request as untenanted and reject tenant-scoped operations; no fallback to unsigned body/header/query tenant.
• Add v0.2 release note warning SigningContext.tenant() may be null until v0.3 tenant resolver wiring lands.

References:

• PR docs: update roadmap for AdCP 3.1 parity #23
• ROADMAP.md D22 / Track 4
• specs/signing-context.md