fix: silence -Wnonnull-compare and Botan FFI deprecation warnings

[ronaldtse]

Summary

Fixes two build-warning classes reported in the v0.18.1 release build (GCC).

1. -Wnonnull-compare in src/librekey/rnp_key_store.cpp

RNP_LOG_KEY(msg, &srckey) expanded to if (!(&srckey)), but &srckey is the address of a Key& reference and is provably non-null, so GCC flagged every such call site. Added an RNP_LOG_KEY_NN variant (no null check) in src/lib/logging.h and switched all &srckey call sites to it. Nullable-pointer sites (primary, added_key) keep the null-checking RNP_LOG_KEY. Verified there are no remaining RNP_LOG_KEY(..., &ref) call sites in the tree.

2. -Wdeprecated-declarations in src/lib/crypto/ec.cpp and src/lib/crypto/eddsa.cpp

botan_privkey_x25519_get_privkey, botan_pubkey_x25519_get_pubkey, and botan_privkey_ed25519_get_privkey are deprecated in Botan 3.x. Switched to the generic botan_privkey_view_raw / botan_pubkey_view_raw API (available since Botan 3.6) under a BOTAN_VERSION_CODE >= 3.6.0 guard, keeping the legacy calls for older Botan (2.x and 3.0–3.5, where the helpers are not deprecated and view_raw does not exist). A small length-checked view callback (rnp_botan_view_bin) and descriptor (rnp_botan_view_buf) live in src/lib/crypto/botan_utils.hpp; the per-site descriptors are declared inside the #if so they don't trigger -Wunused-variable on older Botan.

A codebase-wide sweep confirmed the remaining deprecated Botan FFI calls (botan_key_wrap3394/botan_key_unwrap3394 in ecdh.cpp, the _enc SM2 loaders) are already behind CRYPTO_BACKEND_BOTAN3 guards or use the non-deprecated overloads, and that ed448/x448 paths do not use the deprecated getters.

Notes

• Builds against Botan 2.x and 3.0–3.5 are unaffected (legacy call paths retained).
• The pre-existing Botan 3.12 native C++ API breakage in ec.cpp (around Botan::EC_Group::from_name, under ENABLE_CRYPTO_REFRESH) is unrelated to these warning classes and intentionally left out of scope.

Test plan

• Configured with -DCRYPTO_BACKEND=botan3 against Botan 3.12; librnp-obj, rnpkeys, and rnp build with zero warnings/errors (Apple Clang 15).
• ec.cpp, eddsa.cpp, rnp_key_store.cpp compile clean.
• Runtime: generated an Ed25519 primary + Curve25519 ECDH subkey (expert mode option 22) — exercises all three migrated view_raw call sites.
• Sign/verify with the Ed25519 key passes; encrypt/decrypt round-trip through the Curve25519 subkey passes.
• CI (Linux/GCC) confirms -Wnonnull-compare and -Wdeprecated-declarations are gone for these files.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.37%. Comparing base (975b67c) to head (01bbed9).

Files with missing lines	Patch %	Lines
src/librekey/rnp_key_store.cpp	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2446      +/-   ##
==========================================
- Coverage   85.37%   85.37%   -0.01%     
==========================================
  Files         126      126              
  Lines       22790    22775      -15     
==========================================
- Hits        19458    19445      -13     
+ Misses       3332     3330       -2     

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[ronaldtse]

Closing — superseded by #2448 which migrates Botan backend to FFI, resolving the deprecation warnings this PR was silencing. Combined to reduce review overhead.