Add error correction (ECW) support per BIP-93

[kiwihodl]

Summary

Implements BIP-93 Error Correction Wallet (ECW) capability:

• Up to 4 substitution errors correctable
• Up to 8 erasure errors correctable (when positions are known)
• User confirmation required before applying corrections (per BIP-93)

Implementation Approach

Uses validated brute-force search rather than raw BCH decoding:

• Generates correction candidates
• Validates each using the codex32 library's checksum
• Guarantees correctness (every candidate is verified)
• Safer for Bitcoin seed handling

New Files

File	Purpose
src/gf32.py	GF(32) Galois Field arithmetic
src/error_correction.py	Error correction search algorithm
src/bch_decoder.py	BCH algorithms (reference implementation)
tests/test_gf32.py	18 field arithmetic tests
tests/test_correction.py	22 edge case tests
tests/test_bch.py	BCH decoder unit tests

Test Coverage

51 total tests covering:

• Happy path (valid strings, 1-2 error correction)
• Edge cases (empty, whitespace, None, invalid chars, wrong length)
• Position boundaries (start, end, prefix)
• Boundary conditions (max_errors=0, clamping)
• Erasure edge cases (negative pos, out of range, too many)
• Utility functions

Test Results

=== test_vectors.py ===
vector2: seed OK
vector3: seed OK

=== test_256bit.py ===
All 256-bit tests passed! (9 tests)

=== test_gf32.py ===
All GF(32) tests passed! (18 tests)

=== test_correction.py ===
All 22 tests passed!

How to Test

cd codex32_terminal
source venv/bin/activate
python tests/test_vectors.py
python tests/test_256bit.py
python tests/test_gf32.py
python tests/test_correction.py

🤖 Generated with Claude Code