docs(build): record the thin-binary layout and its measured effect

Replace the 'known remaining inefficiency' section with the layout that
now exists, plus the measured before/after (clean release build: 2m14s
-> 1m46s wall, keydr units 61.0s -> 32.4s, peak ~2.0 GB unchanged).

Add a maintenance note: new modules go in src/lib.rs, not src/main.rs.
Re-declaring them in the binary would silently restore the double
compile this refactor removed.

docs/BUILDING.md

@@ -9,8 +9,9 @@ cargo build --release
 ```
 
 Plain cargo. No wrapper, no memory ceiling, no reduced optimisation.
-A clean release build peaks at **1.92 GB** and takes **2m17s** on a
-4-core / 7.6 GB VM.
+A clean release build peaks at **~2.0 GB** and takes **1m46s** on a
+4-core / 7.6 GB VM; an incremental rebuild after touching one file is a
+few seconds.
 
 If that's not what you're seeing, read on.
 
@@ -87,35 +88,38 @@ Nothing there reduces the optimisation of shipped code.
 
 ---
 
-## Known remaining inefficiency
+## Project layout
 
-`src/main.rs` re-declares the whole module tree (`mod app; mod config;
-...`) instead of depending on the `keydr` lib target, so **every module
-compiles twice** — once for the lib, once for the bin. The `i18n!` macro
-must therefore be invoked in both crate roots (`t!()` expands to
-`crate::_rust_i18n_t`, so it must exist in each crate).
+All application code lives in the **library** (`src/lib.rs` → `src/run.rs`
+and the module tree). `src/main.rs` is a 10-line entry point that calls
+`keydr::run()`.
 
-Measured with `cargo build --release --timings` on a clean tree, the two
-duplicated units are the slowest in the whole graph:
+This matters for build time. `main.rs` used to re-declare the whole module
+tree (`mod app; mod config; ...`), which made the binary a *second
+independent crate*: every module — and the entire rust-i18n translation
+table — was compiled twice, and 281 unit tests ran twice.
+
+Measured effect of consolidating it, clean release build:
 
 ```
-  33.7s  keydr  (bin)
-  26.0s  keydr  (lib)
-   1.4s  generate_test_profiles (bin)
-  ------
-  61.0s of 292.2s total unit-seconds (21%)
+                      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%)
 ```
 
-So restructuring `main.rs` to `use keydr::...` would save roughly 26s of
-compile work — real, but nowhere near half the build. Wall-clock saving is
-smaller still, since those units partly overlap with dependency
-compilation across 3 parallel jobs (292s of unit-work compressed into
-~2m15s wall). Worth doing for code hygiene; not a build-performance
-emergency.
+Test coverage is unchanged — 341 unique tests before and after. The
+execution count dropping from 622 to 340 is the duplicate run disappearing.
 
-Note the remaining ~79% is dependency compilation (`reqwest` 12.7s,
-`clap_builder` 11.4s, `toml_edit` 9.3s, `tokio` 8.0s...), which is
-one-time work that incremental builds skip entirely.
+**Keep it this way:** if you add a module, declare it in `src/lib.rs`, not
+`src/main.rs`. Re-declaring modules in the binary would silently restore
+the double compile.
+
+The remaining ~79% of build time is dependency compilation (`reqwest`
+12.7s, `clap_builder` 11.4s, `toml_edit` 9.3s, `tokio` 8.0s...), which
+incremental builds skip entirely.
 
 ---