SHA3-05 SP 800-185 encoding helpers

[turkananation]

SHA3-05 SP 800-185 Encoding Helpers

Summary

Implement and test the SP 800-185 encoding helpers:
left_encode, right_encode, encode_string, bytepad, and test-only
substring behavior.

Scope

• Add lib/src/common/sp800_185.dart.
• Implement byte-oriented helper APIs.
• Add focused tests for zero, ordinary values, boundary values, and invalid
  widths.
• Keep bit-string complexity isolated unless public API review says
  otherwise.

Acceptance criteria

• Encoding helpers match SP 800-185 examples and derived-function vectors.
• Invalid widths or unsupported input shapes throw before output is
  produced.
• cSHAKE work does not start until helper tests are green.

References

• doc/FIPS202_SP800185_RELEASE_GUIDE.md - Encoding helpers
• SP 800-185 Section 2.3

Priority: P0