Opaque and stable errors

[yukibtc]

Description

Use a single opaque error type per crate, backed by a small and stable ErrorKind enum. This keeps the public error API stable while allowing internal error sources to change without breaking downstream users.

Checklist

• I followed the contribution guidelines
• I updated the CHANGELOG (if applicable)
• I personally wrote and understood all code in this PR

[codecov]

Codecov Report

❌ Patch coverage is 58.10089% with 706 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.21%. Comparing base (b46813e) to head (3c25983).
⚠️ Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
sdk/src/error.rs	47.95%	51 Missing ⚠️
crates/nostr/src/nips/nip47.rs	16.98%	44 Missing ⚠️
crates/nostr/src/event/builder.rs	17.39%	38 Missing ⚠️
crates/nostr-keyring/src/error.rs	0.00%	37 Missing ⚠️
crates/nostr/src/nips/nip34.rs	20.51%	31 Missing ⚠️
database/nostr-ndb/src/lib.rs	0.00%	31 Missing ⚠️
sdk/src/relay/inner.rs	29.72%	26 Missing ⚠️
crates/nostr/src/nips/nip60.rs	22.58%	24 Missing ⚠️
crates/nostr/src/nips/nip46.rs	32.35%	23 Missing ⚠️
signer/nostr-connect/src/error.rs	0.00%	23 Missing ⚠️
... and 88 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1375      +/-   ##
==========================================
+ Coverage   72.99%   75.21%   +2.22%     
==========================================
  Files         202      198       -4     
  Lines       33280    32439     -841     
==========================================
+ Hits        24292    24399     +107     
+ Misses       8988     8040     -948     

Flag	Coverage Δ
rust	75.21% <58.10%> (+2.22%)	⬆️

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