fix: Allow following specialized rune level plans

With recent changes to stair exploration in a18a5b1, branch-specific
plans for navigating to the rune for Swamp were no longer properly
triggered. Instead the level exploration code was resetting stair
exploration, which made it hard for qw to make much progress, since it
had to travel up and try stairs before proceeding further into the cloud
generators. This commit has the exploration code not perform a reset if
we've autoexplored the rune level and found all upstairs, which will
allow the Swamp plans to take over until (hopefully) the rune is found.

source/branches.lua

@@ -243,6 +243,10 @@ function in_hell_branch()
 end
 
 function branch_rune_depth(branch)
+    if not branch_rune(branch) then
+        return
+    end
+
     if branch == "Abyss" then
         return 3
     else

source/gameplans.lua

@@ -531,7 +531,7 @@ function explored_level(branch, depth)
     return autoexplored_level(branch, depth)
         and have_all_stairs(branch, depth, DIR.DOWN, FEAT_LOS.REACHABLE)
         and have_all_stairs(branch, depth, DIR.UP, FEAT_LOS.REACHABLE)
-        and (depth < branch_rune_depth(branch) or have_branch_runes(branch))
+        and (have_branch_runes(branch) or depth < branch_rune_depth(branch))
 end
 
 function explored_level_range(range)
@@ -845,8 +845,14 @@ function finalize_exploration_depth(branch, depth)
         if depth_up_finished then
             if depth_down_finished then
                 if down_unreach then
-                    level_stair_reset(branch, up_depth, DIR.DOWN)
-                    level_stair_reset(branch, depth, DIR.UP)
+                    -- We don't try stair resets if we're still looking for the
+                    -- rune. This way we'll instead try branch-end-specific
+                    -- plans for e.g. Swamp.
+                    if have_branch_runes(branch)
+                            or depth < branch_rune_depth(branch) then
+                        level_stair_reset(branch, up_depth, DIR.DOWN)
+                        level_stair_reset(branch, depth, DIR.UP)
+                    end
                     return depth
                 end
 

source/variables.lua

@@ -215,7 +215,6 @@ local plan_explore
 local plan_explore2
 local plan_move
 
-
 local plan_orbrun_rest
 local plan_orbrun_emergency
 local plan_orbrun_move