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, 2 insertions, 2 deletions

diff --git a/docs/lsp.md b/docs/lsp.md
index 2a4c8784..91d07231 100644
--- a/docs/lsp.md
+++ b/docs/lsp.md
@@ -5,7 +5,7 @@ Three things landed together, and only the first two are permanent:
 1. **An async execution model.** The core stays a state machine; slow work goes
    to a worker and comes back as an event.
 2. **A helix-exact keymap** for every LSP command.
-3. **A seam** (`src/lsp.zig`) with exactly one function behind it, so competing
+3. **A seam** (`src/lsp/lsp.zig`) with exactly one function behind it, so competing
    backends can be swapped, measured, and thrown away.
 
 ## The async model
@@ -126,7 +126,7 @@ document — which matters precisely when the pane you are in is the problem.
 
 ## Writing a backend
 
-`src/lsp.zig` is the seam. An implementation supplies three things and touches
+`src/lsp/lsp.zig` is the seam. An implementation supplies three things and touches
 nothing else:
 
 ```zig