feat(digest): port Digest::SHA3 via Bouncy Castle; refactor NetSSLeay key parsing

[fglock]

Summary

Ports Digest::SHA3 to PerlOnJava via Bouncy Castle, and uses the
newly-added dependency to clean up private-key DER parsing in
NetSSLeay.java. See dev/modules/digest_sha3.md for the full
design rationale.

Before this change, ./jcpan -t Digest::SHA3 installed the CPAN
distribution but failed 11/14 tests with Undefined subroutine &Digest::SHA3::newSHA3 etc., because Digest::SHA3 is an XS module
and PerlOnJava had no Java-side port.

What's in this PR

1. Dependency: adds bcprov-jdk18on + bcpkix-jdk18on v1.78.1
   in gradle/libs.versions.toml and build.gradle.

2. DigestSHA3.java (new): BC-backed Keccak wrapper
   implementing the full XS interface of CPAN Digest-SHA3-1.05
   (newSHA3, shainit, sharewind, shawrite, add, digest,
   hexdigest, b64digest, squeeze, clone, hashsize,
   algorithm, and the 18 one-shot functions from sha3_224 through
   shake256_base64).

   • Subclasses KeccakDigest directly and applies the SHA-3 ("01")
     and SHAKE ("1111") domain separators in Java — SHA3Digest.doFinal
     and SHAKEDigest.doOutput call absorbBits internally, which
     fails after we've already flushed a non-byte-aligned reservoir.
   • Bit reservoir supports multi-call non-byte-aligned add_bits —
     CPAN bitorder.t does ->add_bits("0")->add_bits("1")->add_bits("1")
     which BC cannot handle through absorbBits alone.
   • No Perl shim needed: the unmodified CPAN lib/Digest/SHA3.pm
     calls XSLoader::load('Digest::SHA3') which dispatches to our
     Java class via XSLoader.java.

3. NetSSLeay.parsePrivateKeyDer refactor: replaces a
   trial-and-error loop over {RSA, EC, DSA, EdDSA} KeyFactory
   calls plus a hand-rolled PKCS#1→PKCS#8 DER wrapper with
   PrivateKeyInfo.getInstance + JcaPEMKeyConverter. Auto-detects
   the algorithm from the DER AlgorithmIdentifier and now supports
   Ed25519/Ed448 keys for free. Deletes wrapPkcs1InPkcs8 and its
   local DER helpers.

4. Doc updates:

   • dev/modules/digest_sha3.md (new): design + implementation log.
   • dev/modules/netssleay_complete.md: resolves the "adopt Bouncy
     Castle?" open question and updates the runtime-dependencies
     section.

Explicitly out of scope (future PRs)

• Encrypted-PEM write path with traditional SSLeay format (stubbed today).
• DH parameter PEM parsing.
• PKCS#12 with non-standard MACs.
• CSR builder / X509 extension DER rewrite via
  JcaPKCS10CertificationRequestBuilder and org.bouncycastle.asn1.*.
  The remaining hand-rolled DER code is extensively used and fully
  tested; a mechanical BC port deserves its own focused PR.
• Fixing Digest::SHA.add_bits partial-byte handling (BC doesn't
  expose bit-level SHA-2 primitives; would need reflection or a
  hand-rolled compression loop).
• New CPAN crypto ports now unblocked: Digest::Keccak,
  Digest::BLAKE2, Digest::BLAKE3, Digest::HMAC_SHA3_*,
  Crypt::OpenSSL::AES, Crypt::CBC backends, Crypt::JWT.

Test plan

• ./jcpan -t Digest::SHA3 → 14/14 test files pass (33 subtests),
  including t/bit-sha3-{224,256,384,512}.t,
  t/bit-shake{128,256}.t, t/bitorder.t, t/sha3-{224,256,384,512}.t.
• prove -e ./jperl src/test/resources/unit/netssleay_*.t →
  2553/2553 tests pass (no regressions in the BC-based
  parsePrivateKeyDer).
• make → all unit tests green.

Size impact

• +8 MB for bcprov-jdk18on, +3 MB for bcpkix-jdk18on. Modest
  compared to the existing icu4j (~12 MB).
• Runtime dependency, MIT-style license (compatible with Apache 2.0).
• Not a FIPS build — if FIPS is ever required, switch to bc-fips.

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