DeskTop.system: Prevent crash copying to RAMCard

The DeskTop.system launcher had a latent issue where the last bit of
the code ended up overwritten by the copy buffer when copying to
RAMCard. An assert would have caught this but wasn't present.
Previously, we got lucky - it was the tail end of a proc that would
only happen if non-standard files were present to copy.

The change in e83915e pushed more code into the danger zone, and so a
crash was inevitable. Oops! Fix by adjusting buffer start location and
adding an assert to catch this in the future.

desktop.system/desktop.system.s

@@ -21,7 +21,7 @@
 ;;;          :.............:
 ;;;          |.............|
 ;;;          |.(buffer)....|    * data buffer for copies to RAMCard
-;;;   $3680  +-------------+
+;;;   $3900  +-------------+
 ;;;          |             |
 ;;;          |             |
 ;;;          |             |
@@ -55,7 +55,7 @@ src_io_buffer   := $E00         ; 1024 bytes for I/O
 dst_io_buffer   := $1200        ; 1024 bytes for I/O
 selector_buffer := $1600        ; Room for `kSelectorListBufSize`
 
-copy_buffer     := $3700
+copy_buffer     := $3900
 kCopyBufferSize = MLI - copy_buffer
         .assert (kCopyBufferSize .mod BLOCK_SIZE) = 0, error, "better performance for an integral number of blocks"
 
@@ -2279,3 +2279,7 @@ done:   rts
         ADJUSTCASE_VOLBUF  := $820
         ADJUSTCASE_IO_BUFFER := src_io_buffer
         .include "../lib/adjustfilecase.s"
+
+;;; ============================================================
+
+        .assert * <= copy_buffer, error, "copy_buffer collides with code"