build system (first pass)

TODO.md

@@ -710,7 +710,16 @@
   - native function pointers are non-variadic v1; C variadic function pointers stay
     under `*c_func(...) R`
 
-28. brolang build system (requires comptime execution)
+28. brolang build system — v0 shipped
+  - `brolang build [root]` reads a declarative `config` constant from
+    `root/build.bro` (importing `@std/build`'s `BuildConfig`) and compiles the
+    program package it names. The config is read from the checked HIR — build.bro
+    is never lowered — so it is literal-only.
+  - enabled `&<array literal>` (Zig's `&.{...}`): the literal is promoted to an
+    anonymous global whose address decays to a slice, so list fields like
+    `libraries = &["raylib"]` work; empty lists are `&[]`
+  - deferred: build graph / steps / caching, multiple artifacts, computed paths
+    (needs string building), struct field defaults to drop `&[]` on empty lists
 
 ## A word on multi-unwrap