Harden discriminant and proof bounds validation

[hoffmang9]

Summary

• add explicit runtime bounds checks for discriminant size and form serialization in verifier and BQFC paths, replacing assert-only assumptions on externally derived inputs
• harden vdf_client session parsing for iteration header/body and stop ACK handling with deterministic error returns on malformed network data
• align discriminant-size validation to BQFC_MAX_D_BITS and add DiscriminantBoundsRegressionTest coverage for oversized discriminant/serialization inputs

Test plan

• cmake -S src -B build-security-check -DBUILD_PYTHON=OFF -DBUILD_CHIAVDFC=OFF -DBUILD_VDF_CLIENT=OFF -DBUILD_VDF_BENCH=OFF -DBUILD_VDF_TESTS=ON -DBUILD_HW_TOOLS=OFF
• cmake --build build-security-check --target regression_unit_tests regression_io_tests
• ctest --test-dir build-security-check --output-on-failure -R '^regression\.'
• performance check (vdf_bench square 250000, 5 runs baseline vs patched): median delta ~ -0.1%
• performance check (vdf_bench square_asm 250000, 5 runs baseline vs patched): median delta 0.0%

Made with Cursor

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Note

Medium Risk
Touches proof verification and quadratic-form serialization boundaries; incorrect bounds could cause valid proofs to be rejected or trigger new error paths, but changes are primarily defensive input validation.

Overview
Hardens bounds checking for discriminants and form serialization. bqfc_* routines now reject invalid d_bits/oversized forms at runtime (replacing assert assumptions), with bqfc_export returning errors on insufficient buffers and bqfc_serialize/bqfc_get_compr_size validating sizes against BQFC_MAX_D_BITS/BQFC_FORM_SIZE.

Tightens verifier and discriminant-size validation. The verifier now enforces disc_size_bits and discriminant bit-length ranges before processing proofs, CreateDiscriminant aligns its max to BQFC_MAX_D_BITS, and new regression tests cover oversized discriminant bits plus BQFC serialize/deserialize rejection.

Improves network input parsing in vdf_client. Iteration header/body parsing now validates decimal digits, length, and uint64_t overflow, and STOP ACK handling now throws on malformed responses instead of relying on assert.

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[hoffmang9]

@cursor review

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[hoffmang9]

Cursor didn't find anything when run again. I pushed the tools to make sure these didn't hit performance, and logically they do not.