docs: Update uploadSTBLogs design docs to reflect SHA256 archive integrity logging

[Copilot]

The SHA256 archive integrity logging feature (added to the Direct upload path) was not reflected in any design or requirements documentation.

Changes

• Requirements (uploadSTBLogs_requirements.md)

  • Promotes OpenSSL EVP from optional to required dependency (SHA256)
  • Updates CPU constraint: SHA256 once per Direct upload; MD5 only when encryption flag set
  • Adds observability requirement: log SHA256 at INFO level before each Direct upload

• HLD (uploadSTBLogs_HLD.md)

  • Inserts SHA256 calculation as step 2 of Upload Execution Steps (Direct path only)
  • Adds section 8a "Archive Integrity (Direct Path)" describing purpose and approach

• LLD (uploadSTBLogs_LLD.md)

  • Adds calculate_file_sha256 to functional partitioning table
  • Adds section 7a with full function signature, implementation notes (EVP pipeline, ferror check after read loop, nibble-lookup hex encoding, output_size ≥ 65 guard), and inline code showing call site in execute_direct_path()

• Flowchart (uploadSTBLogs_flowcharts.md)

  • Adds "Calculate & Log SHA256 (Direct Path Only)" node between pre-sign success and S3 PUT in the fallback sub-flow diagram

• Sequence diagram (uploadSTBLogs_sequence.md)

  • Adds HashUtil participant and SHA256 calculation/log steps to the Normal Path sequence