refactor: move the TUI into the library so it compiles once

src/main.rs re-declared the entire module tree (`mod app; mod config;
...`), which made the binary a second independent crate. Every module --
and the whole rust-i18n translation table generated by `i18n!` -- was
compiled twice, and 281 unit tests were executed twice.

Move all application code to src/run.rs inside the library, exposed as
`keydr::run()`. main.rs is now a 10-line entry point.

Mechanical changes only; no logic was altered:
- src/run.rs is src/main.rs minus the crate-root preamble
- `fn main()` -> `pub fn run()`; `struct Cli` -> `pub struct Cli`
- bare `use app::...` / inline `ui::theme::Theme` references are now
  qualified with `crate::` (19 imports, 342 inline references), since
  they are no longer resolving from the crate root

Clean release build, measured with --timings:

                        before      after
  keydr (bin)            33.7s       0.3s
  keydr (lib)            26.0s      31.0s
  keydr total            61.0s      32.4s   (-28.6s)
  total unit-seconds    292.2s     264.5s
  wall clock            2m14s      1m46s    (-28s, -21%)

Test coverage is unchanged: 341 unique tests before and after (verified
by diffing `--list` output, not just counts). The apparent drop from
622 to 340 executions is the duplicate run disappearing -- 281 tests
previously ran once per crate copy.

Verified the binary still works: --help/--version intact, TUI renders
its welcome screen under tmux, and translations resolve (checked both
English and German).

Note: `cargo check --benches` fails with 3 errors, but those are
pre-existing on main (NgramStat field renames) and unrelated.

src/lib.rs

@@ -1,12 +1,15 @@
-// Library target exists solely for criterion benchmarks.
-// The binary entry point is main.rs; this file re-declares the module tree so
-// that bench harnesses can import types via `keydr::engine::*` / `keydr::session::*`.
-// Most code is only exercised through the binary, so suppress dead_code warnings.
+// The keydr library. This holds ALL application code, including the TUI
+// event loop and rendering (`run`), so that everything is compiled exactly
+// once. src/main.rs is a thin wrapper that just calls `run()`.
+//
+// Previously main.rs re-declared this same module tree, making the binary a
+// second independent crate: every module -- and the whole rust-i18n
+// translation table generated by `i18n!` -- was compiled twice.
 #![allow(dead_code)]
 
 rust_i18n::i18n!("locales", fallback = "en");
 
-// Public: used by benchmarks and the generate_test_profiles binary
+// Public: used by the binary, benchmarks, and generate_test_profiles.
 pub mod config;
 pub mod engine;
 pub mod keyboard;
@@ -14,9 +17,12 @@ pub mod l10n;
 pub mod session;
 pub mod store;
 
-// Private: required transitively by engine/session (won't compile without them)
+// Internal to the application, but `run` needs them.
 mod app;
 mod event;
 mod generator;
 mod i18n;
+mod run;
 mod ui;
+
+pub use run::run;