Adjust the placement of Ilsuiw (alexjurkiewicz)

Currently Ilsuiw has a 100% chance of placing on Shoals:4 if she didn't
place on Shoals:3. Fighting her will be a more memorable (some would say
'unique') experience if doesn't happen every time on a specific level.
This commit introduces a 1/5 chance that her Shoals:4 guardian vault
places. It also sets the range of her normal unique vault to Shoals:2-3,
both so that she can place on Shoals:2 and so that this vault does not
give an additional chance of placing her on Shoals:4. Finally, we give
her normal unique vault the typical weight of 50 we use for
branch-specific uniques. Now her chances of placement on Shoals:2-3 are
more similar to that of placing on Shoals:4.

After this commit, mapstat shows her chance of placing overall in shoals
is between 40 and 50% (more objstat iterations needed to be sure), with
a slightly higher 20-25% chance of placing on Shoals:4 compared to about
10-15% chance for both Shoals:2 and Shoals:3. This is similar to
placement of other branch-specific uniques like Polyphemus, Arachne, and
Vashnia. Closes crawl#774.

crawl-ref/source/dat/des/branches/shoals.des

@@ -12,7 +12,9 @@ function shoals_finale_setup(e, qualifier, decoy_count)
   e.hook('post_place', function ()
     -- one rune hut, one ilsuiw hut, maybe a mimic hut, and a bunch of decoys.
     dgn.place_maps{tag=("shoal_rune" .. qualifier), count=1, die_on_error=true}
-    dgn.place_maps{tag="shoal_guardian", count=1, die_on_error=true}
+    if crawl.one_chance_in(5) then
+        dgn.place_maps{tag="shoal_guardian", count=1, die_on_error=true}
+    end
     if crawl.one_chance_in(10) then
         dgn.place_maps{tag=("shoal_mimic" .. qualifier), count=1,
                        die_on_error=true}

crawl-ref/source/dat/des/builder/uniques.des

@@ -364,7 +364,10 @@ WEIGHT: 50
 : place_unique(_G, "Polyphemus band")
 
 NAME:   uniq_ilsuiw
-DEPTH:  Shoals:3-
+# Her placement on Shoals:$ is handled through the shoal_guardian tagged
+# vaults.
+DEPTH:  Shoals:2-, !Shoals:$
+WEIGHT: 50
 : place_unique(_G, "Ilsuiw band")
 
 NAME:   uniq_arachne