docs: add crypto checksum PRD

[rebeccapowell]

Summary

• document next phase adding Base58Check, Bech32/Bech32m, and EIP-55 checks
• outline Bitcoin legacy/SegWit address, Lightning invoice, and Ethereum address validators

Testing

• dotnet test -f net8.0

---

https://chatgpt.com/codex/tasks/task_e_68c4908bf5a4832b8d921cb33d4182e9

[github-actions]

Test Results

291 tests  ±0   291 ✅ ±0   0s ⏱️ ±0s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 37ced00. ± Comparison against base commit 3ca57d3.

This pull request removes 9 and adds 9 tests. Note that renamed tests count towards both.

BsnTests ‑ RandomString_IsInvalid(random: "randomeeea30d7-a6c8-4d80-996d-c2928e8c13dd")
BtwTests ‑ RandomString_IsInvalid(random: "random34352bf6-6334-4ecb-8351-c79a79f55da1")
CifTests ‑ RandomString_IsInvalid(random: "random5cfd1368-54d8-4c5d-8ee4-deae7b218684")
CompanyNumberTests ‑ RandomString_IsInvalid(random: "randome47171ee-03d7-4db9-b75a-ae66a223b156")
FrVatTests ‑ RandomString_IsInvalid(random: "random6e5d337e-c041-4712-ab24-aa11930a9fe7")
NieTests ‑ RandomString_IsInvalid(random: "random2db0e2d7-a430-4eda-8c16-45ab7d2b668e")
NifTests ‑ RandomString_IsInvalid(random: "random29ef10cc-9bc9-42b3-9f02-0e11675273f8")
PivaTests ‑ RandomString_IsInvalid(random: "randomf4a444a7-5c52-42a5-986c-f17df5506c70")
UkVatTests ‑ RandomString_IsInvalid(random: "random501c49b8-6d64-4bf4-928f-bccd3601e08b")

BsnTests ‑ RandomString_IsInvalid(random: "random155ac06f-f79f-4770-8df4-7b5dfaaf38ee")
BtwTests ‑ RandomString_IsInvalid(random: "random884be009-c552-42a6-8cae-d30cf534dfff")
CifTests ‑ RandomString_IsInvalid(random: "random11cb3d0d-0716-4024-880c-04e24f897631")
CompanyNumberTests ‑ RandomString_IsInvalid(random: "random4e87f5dc-c97e-4c8e-a9bb-3b7cf14b5776")
FrVatTests ‑ RandomString_IsInvalid(random: "random6d5ea038-b6be-49df-96fb-93c9a54ae133")
NieTests ‑ RandomString_IsInvalid(random: "random30b57541-055d-41c1-9216-242e31dff817")
NifTests ‑ RandomString_IsInvalid(random: "random0ceba940-57f4-47d7-88c3-9d332fa07837")
PivaTests ‑ RandomString_IsInvalid(random: "random5f871572-0812-4484-b91a-3ba113862a35")
UkVatTests ‑ RandomString_IsInvalid(random: "randomad9df77c-b31d-46d6-ab36-f8c37c2780fc")