Support Bech32 and Bech32m

[dipu-bd]

Add Bech32 (BIP-173) and Bech32m (BIP-350) for SegWit/Taproot addresses, Lightning invoices, and Nostr npub. Highest crypto value after Base58.

Architecture fit

• The 8→5-bit regrouping step fits the existing bit engine.
• The substantial work is the human-readable part (HRP) handling and the BCH checksum (Bech32 constant 1, Bech32m constant 0x2bc830a3), plus the 6-character checksum append/verify.
• No dev-dep oracle exists → vendor the BIP-173/BIP-350 test vectors (valid + invalid) into the test file.

Specs: BIP-173, BIP-350

Effort: L

Checklist

• 8→5-bit conversion reusing the engine
• HRP expansion + BCH checksum (both Bech32 and Bech32m constants)
• Vendored BIP valid/invalid vectors as known-answer + invalid-input groups
• Web-safe: verify checksum polymod on node (int semantics)
• README / CHANGELOG / example / test_integration updated
• Format / analyze / test (vm + node) green