structure: backends into src/{tty,gui,lsp}, pane kinds and builtins into their own files

The core now lies FLAT at src/ and every subdirectory is one backend, so a
file being in no directory at all is what says it is core. Pane-kind bodies
leave pardes.zig for term_pane.zig / file_pane.zig / output_pane.zig, leaving
it the layout, the event/effect machine and the generic render loop.

Builtins are one struct each in builtins.zig, and the enum is folded out of
the file's own declaration list at comptime — a zig file IS a struct, so the
list of builtins and the builtins themselves are the same text. Adding one is
writing a struct. Key paths deliberately stay one table for the config pass.

Pure refactor: no golden moved.

1 files changed, 3 insertions, 3 deletions

diff --git a/build.zig b/build.zig
index 87df6727..c11ccccd 100644
--- a/build.zig
+++ b/build.zig
@@ -122,7 +122,7 @@ pub fn build(b: *std.Build) void {
     if (target.result.os.tag == .freebsd or target.result.os.tag == .netbsd or target.result.os.tag == .openbsd)
         root_mod.linkSystemLibrary("util", .{});
 
-    // The language backend (src/lsp_zls.zig) links ZLS as a MODULE — no zls
+    // The language backend (src/lsp/lsp_zls.zig) links ZLS as a MODULE — no zls
     // binary, no JSON-RPC, no protocol. Everything except the browser gets it;
     // emscripten does not, because the web shell has no threads and no-ops the
     // lsp effect anyway, so paying to compile an analyser it can never call
@@ -639,7 +639,7 @@ pub fn build(b: *std.Build) void {
         const crt_test = b.addTest(.{ .root_module = b.createModule(.{
             .target = target,
             .optimize = optimize,
-            .root_source_file = b.path("src/crt.zig"),
+            .root_source_file = b.path("src/gui/crt.zig"),
         }) });
         unit_step.dependOn(&b.addRunArtifact(crt_test).step);
         // deck.zig is pure std too; replay.zig drives it with slices of real
@@ -652,7 +652,7 @@ pub fn build(b: *std.Build) void {
         deck_test.root_module.addImport("deck", b.createModule(.{
             .target = target,
             .optimize = optimize,
-            .root_source_file = b.path("src/deck.zig"),
+            .root_source_file = b.path("src/gui/deck.zig"),
         }));
         unit_step.dependOn(&b.addRunArtifact(deck_test).step);
     }