Bump pinned bitclust to 4ec941b (Windows CI fix)

doc エントリの source_location を相対のまま格納し、Location の
file:line 分割を最後のコロンで行う修正（znz/bitclust 4ec941b）を
取り込む。rake (windows-latest, head) の
「wrong number of arguments (given 3, expected 2)」の対処。

検証: BITCLUST_PATH=/nonexistent（CI と同じ git ソース解決）で
bundle install → rake generate:3.4 成功、
doc/help の source_location が manual/doc/help.md:1（相対）。

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

Author: znz

Gemfile.lock

@@ -1,6 +1,6 @@
 GIT
   remote: https://github.com/znz/bitclust.git
-  revision: 7b32aebf821379d856d8110dadb110f6fe39fc20
+  revision: 4ec941bd3245598d990712bcfc73919ef464f92b
   branch: feature/markdown-conversion
   specs:
     bitclust-core (1.4.0)
@@ -18,16 +18,16 @@ GIT
 GEM
   remote: https://rubygems.org/
   specs:
-    drb (2.2.1)
+    drb (2.2.3)
     json (2.20.0)
-    nkf (0.2.0)
+    nkf (0.3.0)
     progressbar (1.13.0)
-    rack (3.1.21)
+    rack (3.2.6)
     rackup (2.1.0)
       rack (>= 3)
       webrick (~> 1.8)
     rake (13.2.1)
-    webrick (1.8.2)
+    webrick (1.9.2)
 
 PLATFORMS
   ruby