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, 1 insertions, 1 deletions

diff --git a/test/deck/replay.zig b/test/deck/replay.zig
index 9cf829d6..475f7476 100644
--- a/test/deck/replay.zig
+++ b/test/deck/replay.zig
@@ -1,5 +1,5 @@
 //! Replays slices of real Steam Deck recordings (deckcap .zon captures)
-//! through src/deck.zig and checks the aggregate actions, so the deck
+//! through src/gui/deck.zig and checks the aggregate actions, so the deck
 //! control mapping is exercised without the device.
 
 const std = @import("std");