[bigint][bench] Refactor BigInt benches against Python and Rust

[forfudan]

This PR refactors the BigInt benchmarking infrastructure into a cross-language harness that compares decimo.BigInt (Mojo) against Python int (oracle) and Rust num-bigint (compiled peer), and adds a new BigInt performance/correctness planning document.

Changes:

• Replaced the legacy interactive BigInt bench suite with a unified cross-language pipeline (benches/bigint/*) including a new Rust harness and a markdown report aggregator.
• Updated benchmark dispatch/scripts and Pixi tasks to route bigint benches through the new pipeline and modernized report naming to include utc timestamps.
• Added a BigInt enhancement plan document capturing baseline results, lessons learned, and prioritized future work.

[Copilot]

Pull request overview

This PR refactors the BigInt benchmarking infrastructure into a cross-language harness that compares decimo.BigInt (Mojo) against Python int (oracle) and Rust num-bigint (compiled peer), and adds a new BigInt performance/correctness planning document.

Changes:

• Replaced the legacy interactive BigInt bench suite with a unified cross-language pipeline (benches/bigint/*) including a new Rust harness and a markdown report aggregator.
• Updated benchmark dispatch/scripts and Pixi tasks to route bigint benches through the new pipeline and modernized report naming to include utc timestamps.
• Added a BigInt enhancement plan document capturing baseline results, lessons learned, and prioritized future work.

Reviewed changes

Copilot reviewed 25 out of 33 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
pixi.toml	Updates bench/debug tasks to run the new BigInt Mojo harness entrypoint.
docs/plans/bigint_enhancement.md	Adds a consolidated BigInt performance/correctness plan and baseline snapshot.
benches/run_bench.sh	Routes bigint to the new cross-language runner and updates help text.
benches/decimal128/aggregate.py	Renames default report output to include utc timestamp in filename.
benches/bigdecimal/aggregate.py	Renames default report output to include utc timestamp in filename.
benches/bigint/run_all.sh	New cross-language orchestrator (build/run mojo+python+rust, then aggregate).
benches/bigint/aggregate.py	New BigInt CSV-log aggregator that emits side-by-side markdown reports.
benches/bigint/README.md	Documents the new cross-language BigInt benchmark harness layout and usage.
benches/bigint/mojo/bench.mojo	New Mojo-side BigInt benchmark runner emitting per-case CSV logs.
benches/bigint/python/bench.py	New Python-side benchmark runner (oracle) emitting per-case CSV logs.
benches/bigint/rust/src/main.rs	New Rust-side benchmark runner using num-bigint emitting per-case CSV logs.
benches/bigint/rust/Cargo.toml	Adds Rust harness crate configuration and tuned release profile.
benches/bigint/.gitignore	Ignores generated logs/reports and harness build artifacts.
benches/bigint/rust/.gitignore	Ignores Rust target/ output for the harness.
benches/bigint/cases/*.toml	Adds TOML case definitions for each BigInt op bench.
benches/bigint/bench*.mojo, benches/bigint/bench_data/truncate_divide.toml	Removes legacy BigInt bench entrypoints/data superseded by the new harness.

Comments suppressed due to low confidence (1)

benches/run_bench.sh:29

• Help output says omitting the operation gives an interactive menu for the type, but bigint now runs the cross-language pipeline when OP is omitted. This makes the help text inaccurate for bigint (and other cross-language types).

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