Feat : MSM black box implementation

[ocdbytes]

MSM Black Box Support

• Implements black-box Multi-Scalar Multiplication (MSM) in the R1CS compiler with full elliptic
  curve arithmetic over multi-limb field elements.
• Adds a cost model that analytically selects optimal (limb_bits, window_size) parameters,
  following the same pattern as SHA256 spread optimization.
• Implements FakeGLV scalar decomposition for single-point MSM, reducing scalar width by ~50% via
  half-GCD and interleaved two-point windowed scalar multiplication.
• Adds bigint_mod.rs in the prover for 256-bit modular arithmetic (half-GCD, Barrett reduction,
  modular inverse) used in witness solving.
• 18.9% decrease in constraint count as compared to native noir implementation of msm.
• To test MSM blackbox :

  cargo test -p provekit-bench --test compiler -- "embedded_curve_msm"

Performance Comparison :
This is for 2 points call for msm: s_1[p_1] * s_2[p_2]

Metric	Native Noir MSM	Black Box MSM	% Reduction
R1CS Constraints	5,741	4,658	-18.9%
R1CS Witnesses	6,257	5,899	-5.7%
A Entries	6,749	6,813	+0.9%
B Entries	6,364	7,264	+14.1%
C Entries	14,453	9,021	-37.6%
W1 Size	3,354	1,769	-47.3%
W2 Size	3,367	4,130	+22.7%