feat: Some extension field utilities

[Tabaie]

This PR adds convenient I/O methods to extension fields to make GKR code generation in gnark cleaner and more uniform.

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Note

Medium Risk
Touches low-level field encoding and arithmetic used by FFT/GKR paths; Vector remains aliased but new APIs must stay consistent with fr.Element canonical rules.

Overview
Adds canonical serialization and integer/base-field helpers on extension types E2/E4/E6 across babybear, goldilocks, and koalabear: BytesE*, Marshal / SetBytesCanonical, SetInt64/SetUint64/SetBigInt/BigInt, and embedded-base ops (SetElement, AddElement, SubElement, SubFromElement), with round-trip and non-canonical rejection tests.

Renames the E4 slice type to VectorE4 (keeping Vector as a deprecated alias) and wires serialization/FFT call sites to BytesE4 and VectorE4. VectorE6 gains SetRandom / MustSetRandom.

Introduces koalabear polynomial (dense MultiLin, Polynomial, pooling) and extensions/polynomial for E6 multilinear/polynomial utilities (MultiLinE6, FoldFromBase, EvaluateBase, PolynomialE6, PoolE6).

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[ivokub]

Please see comments:

• lets not start panicking when we didn't before (it is not programming error, but input error, it is not worth a panic). I think here we can also fix the silent parsing error for extension.SetString (i.e. it fails silently right now)
• in code generation, lets try to avoid f.Name == "koalabear" and instead define properties in the configuration we base the configuration on. Then all the configuration is kept in a single place.

[ivokub]

And I think the code generation we have right now is a bit messy, we have some "IsKoalaBear" in the templates as well, but we really shouldn't. It should be about the properties "SupportsAVX512()" etc which is then computed based on the width etc.

But lets keep it for another PR to clean up, I think there is an open issue for that already.

[Tabaie]

Fair comments. All addressed. Instead of a boolean for polynomial implementations did a slice of which extensions to generate polynomials for, which defaults to empty.

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[ivokub]

Thanks for the changes. Yes, you have addressed my comments, but added Poseidon2 matrix getters, this is extends the scope of this PR significantly (and I didn't check it for now, I think @ThomasPiellard or @yelhousni would be better to review the correctness of the matrices).

[ivokub]

Thanks for addressing all the comments! Perfect now!