build: package the single-file header script for source builds

[kory-io]

Problem

lbug 0.21.0 on crates.io cannot be built from source (LBUG_BUILD_FROM_SOURCE=1 / LBUG_RUST_BUILD_FROM_SOURCE=1).

build.rs now sets BUILD_SINGLE_FILE_HEADER=ON (#ca706e5; 0.20.4 had it OFF). With that on, lbug-src/src/CMakeLists.txt runs scripts/collect-single-file-header.py, and that script reads scripts/headers.txt. Neither file is in the package include list, so the build fails:

make[2]: *** No rule to make target `.../lbug-0.21.0/lbug-src/scripts/collect-single-file-header.py', needed by `src/lbug.hpp'.  Stop.
make[1]: *** [src/CMakeLists.txt/single_file_header.dir/all] Error 2

Fix

• Add lbug-src/scripts/collect-single-file-header.py and lbug-src/scripts/headers.txt to package.include.
• Add matching rerun-if-changed lines, as the comment in build.rs asks.

Verification

1. Before the fix (published 0.21.0 crate): a from-source build fails with the error above, on macOS arm64 with Python 3.14 found.
2. After the fix: with lbug-src symlinked to a v0.21.0 checkout of LadybugDB/ladybug:
   • cargo package --list now includes both files.
   • LBUG_BUILD_FROM_SOURCE=1 cargo package succeeds. Cargo's package verification builds the extracted tarball from source, and out/build/src/lbug.hpp is generated.

A 0.21.1 with this change would let downstream crates that build from source move to 0.21.