refactor(rust): move lib.rs's test module below the code it tests

[PathGao]

Stacked on #434 — it touches the same region of lib.rs, so this is based on that branch rather than master. Once #434 squash-merges, GitHub retargets this to master and the duplicate drops by patch-id.

The problem

lib.rs is laid out helpers → tests → commands:

  21 – 512    helper functions
 513 – 2364   #[cfg(test)] mod tests          1852 lines, 39% of the file
2366 –        mod setup; mod tab_transfer; …
       …      every #[tauri::command] in the application

Opening the file to find convert_markdown or save_file_content means scrolling past 1852 lines of assertions for code you have not reached yet. After:

	before	after
first #[tauri::command]	line 2366	line 518
#[cfg(test)] mod tests	line 513	line 2891

setup.rs already does it this way — its tests sit at the end, below 1117 lines of installer code. This is the file falling in line with the convention its sibling already follows, not a new convention.

It is a pure move

Rust does not care about item order within a module, so nothing needed rewriting. Verified rather than asserted:

• The 1852-line block is byte-identical. diff of the block extracted from HEAD against the block extracted from the result: no output.
• A sorted line-by-line diff of the entire file before vs after differs only by the three-line comment banner and one blank line added at the seam. Nothing else moved, changed, or vanished.

npm test      562 / 562
npm run check 644 files, 0 errors
cargo test    149 passed          (149 before)
cargo clippy  3 warnings          (3 before — same three)
cargo fmt     53 diff hunks       (53 before — all pre-existing, rustfmt is not enforced here)

Why this does not break the source-reading tests

Two tests include_str! this file and search their own source: the preprocessing-step registry (every_convert_markdown_preprocessing_step_is_registered) and #434's raw-buffer fail-safe check. Both locate their subject by string search, not by line offset, so moving a block below them changes nothing. Confirmed by the unchanged cargo test count rather than by reading them.

Not covered

• This is placement only. The audit that produced it also observed that the markdown pipeline inside lib.rs is now ~1157 lines across 27 functions with no AppHandle, State or IO dependencies — pure &str → String, and the most module-shaped thing in the crate. Cutting src-tauri/src/markdown/ out of lib.rs is the change that would actually reduce the file; it is a real refactor with a real review surface and is deliberately not attempted here.
• The banner is a comment, not a mechanism. Nothing stops a future test module being added mid-file. setup.rs and this file now agree by convention only.
• Line numbers in any external notes referring to lib.rs are invalidated. Nothing in the repo does so — the source-reading tests search by string, and no comment cites a line number in this file.

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