The inline mod tests block had grown to roughly 1290 lines, dwarfing
the production code in lib.rs and making the file hard to navigate.
Per CODE_STYLE_GUIDE.md ("Unit test file layout"), large unit-test
modules belong in an external file.
Extract the entire #[cfg(test)] mod tests { ... } block into a sibling
crates/tarball/src/tests.rs and reference it from lib.rs with the
standard:
#[cfg(test)]
mod tests;
This is pure motion — same 27 tests, same imports (use super::*; still
resolves to lib.rs), no behavior change. lib.rs shrinks from 2334 to
1045 lines.
Closes#307.