fix: enforce bech32 address case rules

[newmattock]

Summary:
Enforce Bech32/SegWit address case rules in pybgl address validation.

Related bounty or issue:
BitgesellOfficial/bitgesell#81

What changed:

• Accepts valid all-uppercase Bech32 addresses by matching the HRP case-insensitively before validation.
• Keeps rejecting mixed-case Bech32 addresses before checksum normalization.
• Allows digits in uppercase Bech32 payloads while still checking the uppercase Bech32 charset.
• Adds regression tests for uppercase acceptance and mixed-case rejection.
• Wires the focused regression into the existing top-level tests.py runner.

Validation:

• python3 -m unittest pybgl.test.bech32_address_validation
• python3 tests.py
• git diff --check

Notes:
This is separate from pybgl#3 and does not touch packaging/build behavior.