fix seek(0) and update demos for low-cost seek

src/constants.a

@@ -11,7 +11,7 @@
 ; ...unused...
 ; E000..E3FF - HGR font data
 ; ...unused...
-; E8F2..FFEE - main program code
+; E8CA..FFEE - main program code
 ; FFEF..FFF9 - API functions and global constants available for main program
 ;              code, prelaunchers, transition effects, &c.
 ;              (LoadFileDirect, Wait/UnwaitForVBL, MockingboardStuff, MachineStatus)
@@ -20,9 +20,9 @@
 ; LC RAM BANK 2
 ; D000..D5FF - ProRWTS data
 ; D600..D8B4 - ProRWTS code
-; D8C5..DC86 - ProRWTS glue code
-; DC87..DC8C - backup of stack (during gameplay and self-running demos)
-; DC8D..DC92 - okvs cache (attract state saved across self-running demo)
+; D8C5..DC94 - ProRWTS glue code
+; DC95..DC9A - backup of stack (during gameplay and self-running demos)
+; DC9B..DCA0 - okvs cache (attract state saved across self-running demo)
 ; ...unused...
 ; DFB4..DFFF - (de)acceleration function
 ;
@@ -202,7 +202,7 @@ iCurBlockLo    = $D601               ; constant
 iCurBlockHi    = $D603               ; constant
 launchpatch    = $D853               ; glue.launch.a
 iAddToPath     = $FEA5               ; Roger Rabbit, avoid, use Infiltrator 2 style instead
-itraverse      = $DB31               ; Roger Rabbit, avoid, use Infiltrator 2 style instead
+itraverse      = $DB3F               ; Roger Rabbit, avoid, use Infiltrator 2 style instead
 ldrlo          = $55                 ; constant
 ldrhi          = $56                 ; constant
 namlo          = $57                 ; constant

src/prodos.impl.lc2.a

@@ -218,6 +218,15 @@ ProDOS_enter
 resetval=$f0
 @imp_seek
          jsr   @set_rdwrbuff
+         lda   sizelo
+         ora   ldrlo
+         ora   ldrhi
+         bne   @not_zero
+         sta   treeidx
+         sta   blkidx
+         beq   @link_jmpzp
+
+@not_zero
          lda   sizelo
          lsr
          pha