formula: add hannorein/rebound

[MeteorsLiu]

Summary

• Add hannorein/rebound as an LLAR Formula from the pinned Conan Center 4.4.1 recipe.
• Drive the upstream Makefile-only librebound target through the LLAR Autotools helper and publish Conan’s shared library, complete src/*.h header set, GPL license, and relocatable rebound.pc metadata.
• Preserve the recipe’s Linux-only support and pthread link requirement; no direct dependencies, package options, or comparator are needed.

Evidence

• Conan Center snapshot d034b32b3106d282c32f0a23314b5c6ba1b0fa69 maps 4.4.1 to upstream tag 4.4.1, commit e3671e77b3f397ee9a817e4771fa278ebd48bffc, with matching archive SHA-256 0ae7bdf20750cd0a83c4905a66c3cde211f09ac732ffb4cba9887e68aba5273b.
• The complete upstream tag set is GNU-orderable; the Formula serves 4.4.1 through default 5.1.1. Root Makefile and library/header contract were checked across every tag in that range.
• The Conan Windows-only stdio patch is intentionally not carried because this Formula rejects the recipe’s unsupported Windows and macOS platforms.

Validation

• Exact 4.4.1: fresh build and Conan-derived C consumer pass.
• Exact 4.4.1: repeated run passes the cache-hit consumer path.
• Default selection: 5.1.1 build and consumer pass.
• Exact Linux arm64 runs for 4.4.1 and 5.1.1 pass through LLAR main 9c062aa5c4b1724bfd30cb72f7e0bda136f9c078 in a Linux container.
• 4.4.0 is rejected below fromVer; Darwin is rejected by the verified platform filter.
• Installed rebound.pc passes pkg-config --validate; full pkg-config --cflags --libs rebound output is used for metadata and consumer compilation.
• git diff --check passes.

Closes #236

[fennoai]

Reviewed the Formula selection, build, packaging, pkg-config metadata, and consumer-test contracts against LLAR main 9c062aa5c4b1724bfd30cb72f7e0bda136f9c078 and upstream tags through 5.1.1. A local default-version build and consumer run succeeded, but the installed metadata does not identify the selected release, so version-gated pkg-config consumers reject the default artifact.

[fennoai]

[P1] Publish the selected source version in rebound.pc

This Formula applies from 4.4.1 through the current default 5.1.1, but every artifact writes Version: 4.4.1. With the default build, pkg-config --modversion rebound therefore reports 4.4.1, and both pkg-config --atleast-version=5.0 rebound and --exact-version=5.1.1 fail even though the installed library came from tag 5.1.1. Please derive the .pc version from the checked-out source (the release is recorded in src/rebound.c) or otherwise ensure each selected release publishes its actual version.