Reuse IPA coefficient storage and cache VK expression evaluation

[ValarDragon]

Summary

Extracted from #507 into an independently reviewable PR against main.
This contains only the four Halo2 source changes from the validated combined
snapshot c0eddb54; IFMA arithmetic, affine/prefetch, and point decoding are
not included.

• Transfer owned IPA generator coefficients into the MSM instead of cloning.
  Defer the constant coefficient until Guard consumption, covering every
  consumption and scaling path.
• Reuse generator scalar storage during prepared MSM evaluation.
• Cache an exact constraint-expression evaluation plan in the verification key,
  sharing common operations while preserving expression and root order.

Pinned-key and transcript hashes are unchanged. No proof or protocol changes.

Validation

Local ARM, Rust 1.97.1 release, orbits,unstable-verifier-fingerprint:
275 unit tests, 7 integration tests, and 9 doctests passed; 4 existing doctests
ignored. Formatting and git diff --check passed. Independent Guard, CSE,
dependency, and API review found no blocker. The four Rust files byte-match
the previously validated combined snapshot.

No isolated whole-proof speedup is claimed for this split. The former combined
#507 benchmark included additional optimizations and does not measure this PR
alone. A keygen-warm Apple M4 Max diagnostic measured plan construction at
about 0.38 ms, with 65,672 retained bytes per key, 37,024 scratch bytes per
verification, and 6,176 output bytes per circuit, excluding allocator metadata.
Existing prepared SRS table allocation is unchanged.

API surface

One internal helper is added:
pub(super) MSM::add_owned_g_scalars(&mut self, Vec<C::Scalar>).
There are no downstream public, pub(crate), trait, enum, feature, or existing
signature changes. Private Guard/VK state and derived Debug output change;
the evaluation plan is not exposed or included in pinned/transcript hashes.

The corresponding changes are being removed from #507; that PR retains the
IFMA backend and existing decoder.

[v12-auditor]

Note

Complete: Audit complete. V12 did not find any issues that need review.

Open the full results here.

Analyzed four files, diff c71526e...4a68b68.

[ValarDragon]

Fable reviewed as well, will merge after rc