rename std/build.sx -> modules/compiler.sx (the compiler-API surface)

Per user direction: the low-level abi(.compiler) primitive surface is the
comptime 'compiler' library, so name the file compiler.sx (a peer of build.sx)
instead of the interim std/build.sx — which also frees the 'build' name for the
default build IMPLEMENTATION (default_build + on_build slot), which will live in
modules/build.sx alongside the BuildOptions DSL.

Updated the two example imports + the plan's Phase 5 file-split note. 704/0
both gates.

library/modules/std/build.sx

@@ -1,29 +0,0 @@
-// Phase 5 — sx-driven build pipeline (compiler primitives).
-//
-// These run in the comptime evaluator (`abi(.compiler)`), serviced by
-// `comptime_vm.callCompilerFn`. They are called from the post-link build driver
-// (a callback registered via `set_post_link_callback`), which always runs on the
-// comptime VM (`core.invokeByFuncId`) — the VM, unlike the legacy interpreter,
-// can allocate/grow the `List`s the driver builds (issue 0141).
-//
-// This is the home the sx `default_build` pipeline grows into; for now it exposes
-// the read-only build metadata queries.
-#import "modules/std.sx";
-
-// The C companion object files for this build (`#import c { #source ... }`,
-// compiled to `.o`) and the `#library` link names. The sx driver passes them to
-// the linker. Answered from the compiler's accumulated build state.
-c_object_paths :: () -> List(string) abi(.compiler);
-link_libraries :: () -> List(string) abi(.compiler);
-
-// The object file the compiler emitted for this build. The compiler emits it
-// eagerly; this returns its path (a query, not an action). The sx driver passes
-// it to `link` alongside the C objects.
-emit_object :: () -> string abi(.compiler);
-
-// Link `objects` into `output`, with the given libraries / frameworks / link
-// flags / target triple. The one genuine ACTION primitive — the compiler keeps
-// the proven linker (Option B); the sx driver orchestrates. Not fallible (the
-// build callback isn't): a link failure fails the build directly.
-link :: (objects: List(string), output: string, libraries: List(string),
-         frameworks: List(string), flags: List(string), target: string) abi(.compiler);