BIP-0085: Add ECC GPG Keys application 828366' (separate from RSA)

[3rdIteration]

Summary

Adds ECC GPG Keys as a new, separate application 828366' for deterministic derivation of OpenPGP ECC keys. This preserves backward compatibility with existing RSA implementations (app 828365') while adding support for:

• Curve25519 (Ed25519/X25519) — key_type 0
• secp256k1 — key_type 1
• NIST P-256/P-384/P-521 — key_type 2
• Brainpool P-256/P-384/P-512 — key_type 3

Key decisions

• Separate app number: ECC uses 828366' (sequential after RSA's 828365'), keeping RSA paths completely unchanged
• key_type 0–3: Remapped from the unified scheme (1–4) since this is an ECC-only application
• Scalar derivation: Direct HMAC truncation for curves ≤64 bytes; BIP85-DRNG only for NIST P-521
• Timestamps: Fixed to Bitcoin genesis block time 2009-01-03 18:15:05 UTC (epoch 1231006505)

Changes

• New ===ECC GPG Keys=== section in bip-0085.mediawiki with full specification, key type table, scalar derivation algorithm, primary/subkey scheme, and test vectors for all 8 curve configurations
• Changelog updated: version 2.1.0 (additive, non-breaking)

Test vectors

All 8 ECC curves tested against the standard BIP85 master key with fresh fingerprints computed via OpenPGP v4 packet construction.