perf(prover): collapse R1CS row-covectors into one before WHIR

[BornPsych]

Summary

R1CS row-covectors a, b, c (~32 MB each at m=20) were each wrapped in their own PrefixCovector and passed to whir.prove as three separate linear forms. WHIR's prepare_and_sumcheck then sampled one FS challenge and folded them into a single covector internally. The three intermediate dense vectors stayed resident across the setup until WHIR consumed them, even though only the combined linear form had any
downstream use.

This PR moves the row-RLC into provekit: sample inner_rlc from the prover transcript right after the three matrix evaluations are sent, fold a, b, c into a single PrefixCovector via the new build_combined_prefix_covector helper, and pass one LF to whir.prove instead of three. Verifier mirrors at the same transcript position; soundness follows from inner_rlc being bound to all three evals before being drawn.

Same direct-RLC applies to the dual-commit path with one shared inner_rlc across both commitments.

WHIR itself is unchanged — the LF count it receives just shrinks from three to one. geometric_challenge(1) short-circuits to [ONE] with no extra FS sample, so the only transcript-level change is the new inner_rlc sample on the provekit side.

Measured impact

Single run, complete_age_check. Baseline used was v1 + the perf(prover): drop R1CS after sumcheck … commit (raised separately). Numbers are for the eventual merged state of both PRs.

Metric	baseline	this PR	Δ
Run peak memory	816 MB	766 MB	−50 MB / −6.1%
prove_with_toml duration	1.96 s	1.93 s	within noise
Allocations	3.56M	3.56M	0

Removes three now-dead helpers from prover/src/whir_r1cs.rs (create_weights_and_evaluations, compute_evaluations, compute_public_weight_evaluation), and refactors the public-inputs-hash absorption into read_public_inputs_challenge so the public-weight covector is built lazily only when needed.

Test plan

• cargo test -p provekit-bench --test compiler --release — 16 passed (incl. complete_age_check, test_public_input_binding_exploit)
• provekit-cli prove → provekit-cli verify round-trip on complete_age_check
• Dual-commit circuits (small-sha, read_write_memory, etc.) round-trip
• Bench additional circuits (t_add_dsc_1850, t_add_id_data_1850) to confirm percentage scales

Rollback

Revert the single commit. The protocol modification is local: one
extra verifier_message() sample in provekit's prover/verifier, no
WHIR-side changes.