Performance: cache paramsQl for HYBRID KeySwitchDown

[amneeksingh]

This PR implements the TODO in keyswitch-hybrid.cpp by avoiding repeated paramsQl construction in the HYBRID keyswitch path.

Changes

• Precompute m_paramsQlHybrid in CryptoParametersRNS::PrecomputeCRTTables() for the first 1..sizeQ towers of Q when HYBRID key switching is used.
• Add GetParamsQlHybrid(sizeQl) in rns-cryptoparameters.h.
• Use the cached params in KeySwitchDown() and KeySwitchDownFirstElement() instead of rebuilding ILDCRTParams on each call.

Notes

• No cryptographic semantics are changed.
• The cache is built during parameter initialization and read afterward.
• No API breaking changes.

[amneeksingh]

(UTGENERAL_ENCODING.packed_int_ptxt_encoding_DCRTPoly_prime_cyclotomics) appears to be pre-existing and unrelated to this PR — it involves prime cyclotomic encoding, not HYBRID KeySwitch or paramsQl caching. Please let me know if you'd like me to investigate further.

[yspolyakov]

(UTGENERAL_ENCODING.packed_int_ptxt_encoding_DCRTPoly_prime_cyclotomics) appears to be pre-existing and unrelated to this PR — it involves prime cyclotomic encoding, not HYBRID KeySwitch or paramsQl caching. Please let me know if you'd like me to investigate further.

Are you saying you saw an error when running this unit test? If yes, please create an issue and copy the error output there.

[yspolyakov]

Oh. I see the output. Let me rerun the CI.