Refactor EC key padding length lookup into shared cryptoECC helper; add BN-254 support

[Copilot]

PR #25’s leading-zero padding logic introduced four duplicated curve-length tables in ECDH/ECDSA, and all four omitted BN-254. This change consolidates curve element-length resolution into a single shared source and makes lookups case-insensitive to avoid drift and mismatches.

• Shared curve element-length source in cryptoECC

  • Added curveElementLengths in src/cryptoECC.js with the full supported set, including "BN-254": 32.
  • Added/exported cryptoECC.curveElementLength(curveName) to normalize via toUpperCase() and return the fixed element byte length.

• Replaced duplicated partLen tables (4 call sites)

  • src/ecdh.js
    • generateKey
    • importKey (p.format === "jwk")
  • src/ecdsa.js
    • generateKey
    • importKey (p.format === "jwk")
  • Each now reads from cryptoECC.curveElementLength(...) instead of inline object literals.

• Targeted BN-254 coverage updates in existing ECC tests

  • Added BN-254 key-generation test cases in test/Test.Ecdh.js and test/Test.Ecdsa.js.
  • Updated test key-length checks to use cryptoECC.curveElementLength(...) directly instead of duplicated ecdhKeyLengths / ecdsaKeyLengths tables.

Example of the lookup change:

// before
var partLen = {
    "P-256": 32, "P-384": 48, "P-521": 66,
    "NUMSP256D1": 32, "NUMSP256T1": 32,
    "NUMSP384D1": 48, "NUMSP384T1": 48,
    "NUMSP512D1": 64, "NUMSP512T1": 64
}[p.algorithm.namedCurve];

// after
var partLen = cryptoECC.curveElementLength(p.algorithm.namedCurve);

[zaverucha]

The test code in Test.Ecdsa.js and Test.Ecdh.js duplicates the tables (ecdhKeyLengths and ecdsaKeyLengths) rather than using cryptoECC.curveElementLength.