Add AES-256 IGE decrypt helper with ARMv8 fast path

[fatshotty]

Summary

• Add DecryptAES256Blocks, with the existing Go implementation as a transparent fallback.
• Use ARMv8 AES instructions on 32-bit ARM Linux and Android only when cgo and the runtime AES capability are available.
• Keep the platform hook private, retain Go 1.15/1.16-compatible build constraints, and resolve getauxval dynamically so older libc/Android targets fall back safely.
• Use a fixed-operation AES S-box for key expansion, avoiding secret-indexed lookup tables.

Verification

• make test
• go vet ./...
• go test ./... with Go 1.16
• Android ARMv7 cgo cross-compile with the local NDK
• Linux ARM cross-compile with CGO_ENABLED=0

The ARM-specific correctness test and benchmark skip when the AES extension is unavailable. They still need to be executed on a real AArch32 ARMv8 device with HWCAP2_AES; no such device is currently connected.

[codecov]

Codecov Report

❌ Patch coverage is 29.41176% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.37%. Comparing base (06dff71) to head (6a4a71e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
aes256_decrypt.go	20.00%	6 Missing and 6 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #106       +/-   ##
===========================================
- Coverage   92.98%   78.37%   -14.61%     
===========================================
  Files           3        5        +2     
  Lines          57       74       +17     
===========================================
+ Hits           53       58        +5     
- Misses          2        8        +6     
- Partials        2        8        +6     

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