Add tank stage 5 with forest cover and a commander tank

Water in stage 4 stops tanks; forest is its opposite - tanks and shells
both pass, but the canopy draws over the tanks so anything sitting in it
is hidden. It needs no collision change at all, only a draw pass after
the tanks.

The enemy roster ends on a commander: 3 hits to kill, faster, fires more
often, worth 300. Kinds carry a worth so the score follows the roster
rather than a flat 100. Crates here can also grant 10s of 3-shot, faster
fire, and five enemies hold the field at once.

Enemy pips shrank to 9px so twelve of them still leave the label room.

Checked headlessly: stage 5 keeps 125 reachable cells and leaves the base
standing in 6 of 10 idle runs; the power badge fires from a crate; and
with the roster reordered in memory the commander pays 300 against 100
for the rest.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

site/index.html

@@ -175,6 +175,7 @@ body{
       <a href="tank.html#2" target="stage" data-key="tank2" title="2단계 미로">2단계</a>
       <a href="tank.html#3" target="stage" data-key="tank3" title="3단계 강철 회랑">3단계</a>
       <a href="tank.html#4" target="stage" data-key="tank4" title="4단계 참호 — 물길·보급 상자">4단계</a>
+      <a href="tank.html#5" target="stage" data-key="tank5" title="5단계 최후 방어선 — 숲·지휘 탱크">5단계</a>
     </div>
   </div>
 
@@ -201,7 +202,8 @@ var PAGES = {
   tank1:   {file:'tank.html#1',    name:'휴게실 · 탱크게임 1단계'},
   tank2:   {file:'tank.html#2',    name:'휴게실 · 탱크게임 2단계'},
   tank3:   {file:'tank.html#3',    name:'휴게실 · 탱크게임 3단계'},
-  tank4:   {file:'tank.html#4',    name:'휴게실 · 탱크게임 4단계'}
+  tank4:   {file:'tank.html#4',    name:'휴게실 · 탱크게임 4단계'},
+  tank5:   {file:'tank.html#5',    name:'휴게실 · 탱크게임 5단계'}
 };
 var DEFAULT = 'korean';