refactor(signature): move ES256K out of the experimental package (RFC 8812)

[Spomky]

Context

ES256K (ECDSA over secp256k1 with SHA-256) is registered by RFC 8812 (August 2020) — alg ES256K and crv secp256k1 in the JOSE registries, "Optional" implementation requirement — and has been "Standards Track" for six years. The library still ships it in the experimental package (src/Experimental/Signature/ES256K.php, Jose\Experimental\Signature\ES256K, registered by signature_experimental.php in the bundle) next to the WebCrypto-only identifiers (RS1, HS1, A*CTR…) that have no RFC. The Experimental label tells users the algorithm may change or go away; RFC 8812 says the opposite.

secp256k1 has full support in the core already: Jose\Component\Core\Util\Ecc knows the curve, ECKey builds it, ES256KeyAnalyzer-style checks exist. Only the algorithm class and its bundle registration are misplaced.

The move is a namespace change, so a BC break for anyone type-hinting or instantiating Jose\Experimental\Signature\ES256K — 5.0.0 material, listed in #717.

Scope

• Jose\Component\Signature\Algorithm\ES256K in src/Library/Signature/Algorithm/, extending ECDSA like ES256 / ES384 / ES512; signature_ecdsa.php registers it in the bundle alongside its siblings.
• In 4.3.x (preparation, no break): keep the experimental class as a thin @deprecated subclass / alias of the new one so both FQCNs resolve to the same service; the bundle registers the algorithm once under the alias ES256K.
• In 5.0.0: delete Jose\Experimental\Signature\ES256K, drop it from signature_experimental.php and from the jwt-experimental split's composer.json description.
• Key analyzer: an ES256KKeyAnalyzer (secp256k1 coordinates length, alg consistency) where ES256KeyAnalyzer lives, if not already present under another name.
• Docs: ES256K moves from the experimental table to the standard signature table, with the RFC 8812 reference and the known limitation that a secp256k1 key exported to PEM cannot be loaded back through KeyConverter on some OpenSSL builds (documented rather than fixed here); changelog and the 5.0.0 upgrade guide.

Out of scope

• The WebCrypto-only algorithms (RS1, HS1, RSA-OAEP-384/512, A*CTR, A*CBC): no RFC, they stay experimental.
• Fixing the secp256k1 PEM round-trip.

Acceptance criteria

• RFC 8812 §3 sign / verify through JWSBuilder / JWSVerifier with the new FQCN, all three serializations; the existing ES256K tests pass unchanged after the namespace swap.
• 4.3.x: a service definition referencing the old FQCN still compiles and produces the same algorithm; a deprecation is triggered at instantiation.
• 5.0.0: the old FQCN is gone, grep -r 'Experimental\\Signature\\ES256K' returns nothing in src/, docs and bundle config.

References

• RFC 8812 §3 (ES256K), §4 (secp256k1 curve), §5 (IANA)
• 5.0.0: remove what 4.3 deprecated, seal what it announced #717 (5.0.0 inventory)