Add tank stage 6: mirrors and a raid on the enemy base

Five stages of holding the line; this one lets you attack. An enemy
eagle sits at the top behind steel, and a player shell that reaches it
ends the stage at once for 1000 - enemy shells cannot touch it, so the
stage cannot clear itself.

Reaching it needs the mirrors. A shell entering one turns 90 degrees,
capped at five turns and stepped one cell past the mirror so it cannot
re-enter the same one. The eagle's only opening is a bullet-only shaft
at column 8, walled in steel and fed by a "/" mirror below it, so only
a shot travelling right along row 6 banks up into it; shooting left or
from underneath deflects away. Mirrors block tanks and cannot be broken,
and they block the enemies' line of sight so they do not waste shots.

The clear/win paths were one block inside afterKill; they are now
clearStage(bonus, note) so the snipe and the last kill share them.

Checked headlessly: the snipe lands and pays 1000, the shaft saw 0 tank
samples against 639 bullet samples, 6 idle runs left the enemy eagle
standing, and closing r11's centre brought the base through 6 of 10
idle runs.

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

site/index.html

@@ -176,6 +176,7 @@ body{
       <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>
+      <a href="tank.html#6" target="stage" data-key="tank6" title="6단계 거울 요새 — 반사판·적 본진 습격">6단계</a>
     </div>
   </div>
 
@@ -203,7 +204,8 @@ var PAGES = {
   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'},
-  tank5:   {file:'tank.html#5',    name:'휴게실 · 탱크게임 5단계'}
+  tank5:   {file:'tank.html#5',    name:'휴게실 · 탱크게임 5단계'},
+  tank6:   {file:'tank.html#6',    name:'휴게실 · 탱크게임 6단계'}
 };
 var DEFAULT = 'korean';