[codex] Bound proof file reads

[ryan-wong-coder]

Summary

• Decode .sproof files through the bounded CBOR reader instead of reading the whole file into memory first.
• Add bounded reads for desktop local proof, global proof, anchor, and single-proof inputs before CBOR type detection.
• Add oversized-input regression coverage for core .sproof and desktop proof-bundle reads.

Root cause

Proof-file readers applied CBOR size limits only after os.ReadFile loaded the entire selected file. A very large local file could therefore consume memory before the existing decode limits had a chance to reject it.

Validation

• go test ./internal/sproof ./clients/desktop
• go test ./...
• go vet ./...