Performance: cache paramsQl for HYBRID KeySwitchDown (implements TODO)

[amneeksingh]

This PR implements the TODO in keyswitch-hybrid.cpp to avoid rebuilding paramsQl on every call in the HYBRID keyswitch path.

Background:
KeySwitchDown and KeySwitchDownFirstElement currently reconstruct paramsQl (ILDCRTParams for the first sizeQl towers of Q) from paramsQlP on every call, allocating vectors and a new ILDCRTParams. This runs in a hot path during relinearization and ApproxModDown.

Changes:

• Precompute and cache m_paramsQlHybrid (params for first 1..sizeQ towers of Q) in CryptoParametersRNS when m_ksTechnique == HYBRID.
• Add getter GetParamsQlHybrid(sizeQl) in rns-cryptoparameters.h.
• Use cached params in KeySwitchDown / KeySwitchDownFirstElement.
• Preserve behavior via fallback to original inline construction when cache is not populated (e.g., after deserialization).
• Factor fallback into a single helper MakeParamsQlFromQlP() to remove duplicated code between the two functions.

Notes:

• No cryptographic semantics are changed.
• Cache is built once during parameter initialization and is read-only thereafter (thread-safe).
• No API breaking changes.

Files changed:

• src/pke/include/schemerns/rns-cryptoparameters.h
• src/pke/lib/schemerns/rns-cryptoparameters.cpp
• src/pke/lib/keyswitch/keyswitch-hybrid.cpp

Happy to add a regression test or benchmark comparison if that would help the review.