Reject Bech32 checksums on witness v1 and later

[SashaMIT]

Summary

decode() treated checksum constant 1 as valid for every witness version. BIP350 keeps Bech32 for v0 and requires Bech32m (0x2bc830a3) for v1 and later. Encoding a 32-byte v1 program with Bech32 still returned (1, [0..31]).

encode() now uses Bech32 for v0 and Bech32m otherwise. bech32_decode() returns (hrp, data, spec). The only caller in this repo is decode().

Test plan

• test_v1_rejects_bech32_checksum is red on master (decode returns the program) and green here
• python3 -m unittest discover -s bitcoin/tests (150 tests OK)
• The three BIP173 v1+ vectors are updated to their Bech32m strings, and the old strings are in bech32_invalid.json

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