[float][gmp] Implement most of the methods for BigFloat

[forfudan]

This PR implements most of the methods for the MPFR-backed BigFloat type (via the existing GMP/MPFR C wrapper) and wires it into the Decimo package, including a dedicated build/run test path and CI coverage for environments with MPFR installed.

Changes:

• Add BigFloat core API: constructors, arithmetic, comparisons, transcendentals, and BigDecimal conversion.
• Extend the C/FFI layer with raw-digit export (mpfr_get_str) to speed up BigFloat → BigDecimal.
• Add BigFloat tests + runner script, plus Pixi tasks and a GitHub Actions job to run them.

[Copilot]

Pull request overview

Implements an MPFR-backed BigFloat type (via the existing GMP/MPFR C wrapper) and wires it into the Decimo package, including a dedicated build/run test path and CI coverage for environments with MPFR installed.

Changes:

• Add BigFloat core API: constructors, arithmetic, comparisons, transcendentals, and BigDecimal conversion.
• Extend the C/FFI layer with raw-digit export (mpfr_get_str) to speed up BigFloat → BigDecimal.
• Add BigFloat smoke tests + runner script, plus Pixi tasks and a GitHub Actions job to run them.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/test_bigfloat.sh	Adds a script to build/link the wrapper and execute BigFloat Mojo tests as binaries.
tests/bigfloat/test_bigfloat.mojo	Adds BigFloat smoke tests for construction, ops, transcendentals, and conversions.
src/decimo/gmp/gmp_wrapper.c	Adds raw-digit export/free helpers using mpfr_get_str/mpfr_free_str.
src/decimo/bigfloat/mpfr_wrapper.mojo	Adds Mojo FFI bindings + doc improvements for MPFR wrapper functions.
src/decimo/bigfloat/bigfloat.mojo	Implements BigFloat including fast to_bigdecimal packing path.
src/decimo/bigfloat/init.mojo	Re-exports BigFloat and aliases from the bigfloat subpackage.
src/decimo/init.mojo	Exposes BigFloat (and aliases) from the top-level decimo package.
pixi.toml	Adds wrapper build + BigFloat test tasks; adjusts doc task invocation.
docs/plans/gmp_integration.md	Expands MPFR/BigFloat design + usage documentation (needs syncing with code).
.github/workflows/run_tests.yaml	Adds a BigFloat CI job (with MPFR installed) and a doc generation job.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.