[Feature] Add Electrum Seed Support to Seed XOR Flow, builds on PR #738

[berlinxray]

Based on: PR #738 (Seed XOR functionality)
Related: PR #513 / #571 (Electrum seed support), Issue #43 (SeedXOR)

Summary

Builds on PR #738 (Seed XOR Flow by @Advaitgaur004) to add Electrum native segwit seed support to the XOR split/combine workflow.

• Electrum seeds can now be used as XOR shards alongside BIP-39 seeds
• XOR recombination auto-detects when the result is an Electrum seed
• The exact Electrum mnemonic is recovered after BIP-39 checksum recomputation
• 13 new unit tests verify Electrum XOR roundtrip, type detection, and validator compatibility
• All 27 XOR tests pass (14 existing + 13 new)

Note: This PR is focused exclusively on Seed XOR + Electrum support. The AES-256-CBC encrypted seed QR import feature is in a separate PR (#882).

Changes (13 files)

Core (src/seedsigner/helpers/mnemonic_generation.py)

• Replaced entropy-based XOR (bip39.mnemonic_to_bytes) with word-index-based approach — Electrum seeds lack valid BIP-39 checksums, so direct entropy extraction fails. Word-index XOR operates on the raw 11-bit indices and reconstructs entropy bits afterwards.

Model (src/seedsigner/models/seed.py)

• Seed._is_electrum_hmac() — HMAC-SHA512 prefix check for segwit ("100") only. The standard prefix ("01") is excluded to prevent false positives during 256-variant brute-force on 24-word seeds.
• Seed.detect_mnemonic_type() — Checks all 16/256 last-word checksum variants for Electrum HMAC match
• Seed.get_electrum_mnemonic() — Recovers the exact last word that makes the mnemonic a valid Electrum seed

Validator (src/seedsigner/helpers/seed_xor_validator.py)

• Use ignore_checksum=True in inverse-shard detection so Electrum seeds don't trigger checksum errors

Controller (src/seedsigner/controller.py)

• Added FLOW__REBUILD_SEEDXOR constant and XOR rebuild methods: validate_rebuild_seedxor_shard(), process_rebuild_seedxor_shard(), remove_rebuild_seedxor_shard(), clear_rebuild_seedxor_data()

Storage (src/seedsigner/models/seed_storage.py)

• Added XOR rebuild state: rebuild_seedxor_shards, rebuild_seedxor_combined_seed, and management methods

Settings (src/seedsigner/models/settings_definition.py)

• Added SETTING__SEED_XOR feature toggle (advanced, enabled by default)

GUI (src/seedsigner/gui/components.py)

• Uncommented CLOSE icon constant used by Seed XOR views

Views (src/seedsigner/views/seed_views.py)

• Complete Seed XOR rebuild flow: RebuildSeedXORLoadShardView, RebuildSeedXORManageView, RebuildSeedXORShowFingerprintView, RebuildSeedXORFinalizeView, RebuildSeedXORFinalizeOptionsView, and more
• "Enter Electrum seed" button gated by SETTING__ELECTRUM_SEEDS
• Auto-detects Electrum vs BIP-39 result, creates ElectrumSeed when appropriate
• Warning screen when Electrum support is disabled but XOR result is Electrum

Scan Views (src/seedsigner/views/scan_views.py)

• ScanSeedQRView accepts is_rebuild_seedxor_shard parameter to route scanned seeds into the XOR flow
• Back button handling for scan screen

Tests

• tests/test_seedxor.py: 14 tests for base XOR logic (combine, commutativity, identity, invalid words, mismatched lengths)
• tests/test_seedxor_electrum.py: 13 tests (roundtrip, type detection, commutativity, validator acceptance, derivation path preservation)
• tests/test_flow_seedxor.py: Integration tests for XOR rebuild UI workflow

Checklist

• pytest passes
• Tested on Raspberry Pi OS (manual build)
• Tested on SeedSigner OS (Pi0)

Test plan

• All 27 XOR tests pass (14 base + 13 Electrum)
• Manual test: Split Electrum seed → enter shares → Rebuild Seed XOR → verify fingerprint matches
• Manual test: Verify "Enter Electrum seed" button only appears when Electrum Seeds setting is enabled
• Manual test: Verify warning screen when XOR result is Electrum but setting is disabled