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.

[MyTH-zyxeon]

Review-assist note for the Bitgesell #81 improvement lane.

I checked the public PR body and diff. This is a focused address-validation correctness fix:

• accepts all-uppercase Bech32/SegWit addresses by matching the HRP case-insensitively before validation
• keeps mixed-case Bech32 payloads rejected before checksum normalization, which matches the normal Bech32 safety rule
• allows numeric characters in uppercase payload validation instead of requiring every payload character to satisfy isupper()
• adds a dedicated regression module and wires it into the existing top-level tests.py runner

Suggested maintainer checks before merge:

• run python3 -m unittest pybgl.test.bech32_address_validation
• run python3 tests.py to confirm the new focused test stays in the default suite
• manually test one lowercase, one uppercase, and one mixed-case Bitgesell SegWit address fixture
• confirm this PR remains independent from packaging/build changes in pybgl#3

From the public patch shape, this looks like a useful low-risk wallet/address correctness candidate for the #81 improvement queue. I did not run wallet, node, RPC, or live-chain actions for this review-assist note.