Use '_' instead of '.' for numbering cosmo escapes

because it gets syntax highlighted weird because '.' is not valid for
like a variable name so the syntax highlighter is splitting the escape
phrase at the '.' which means that phrase won't match when unescaped so
it fails to get unescaped

for example, this would fail:

```
 $$ct
```

posts/2025-06-22-test-nginx-conf-directives.md

@@ -398,11 +398,14 @@ Edit Makefile:
 
     --add-host=git.domain.abc=$(loopback) \
 
+Rebuild image:
 
 ```console
 $ make image-rm image-build
 ```
 
+Run tests:
+
 ```console
 $ make test
 ●●●●

renderers/markdown.moon

@@ -48,7 +48,7 @@ escape_cosmo = (str) ->
 
   cosmo = parse_cosmo! / (tpl) ->
     counter += 1
-    key = "#{dollar_temp}.#{counter}"
+    key = "#{dollar_temp}_#{counter}"
     escapes[key] = tpl
     key
 
@@ -59,7 +59,7 @@ escape_cosmo = (str) ->
 unescape_cosmo = (str, escapes) ->
   import P, R, Cmt, Cs from require "lpeg"
 
-  escape_patt = P(dollar_temp) * P(".") * R("09")^1 / (key) ->
+  escape_patt = P(dollar_temp) * P("_") * R("09")^1 / (key) ->
     escapes[key] or error "bad key for unescape_cosmo"
 
   patt = Cs (escape_patt + P(1))^0 * P(-1)
@@ -84,8 +84,6 @@ write_exec = (cmd, content) ->
 
   fname
 
--- config command like this in site.moon:
--- require("renderers.markdown").cmd = "pandoc --mathjax >"
 class PandocRenderer extends require "sitegen.renderers.html"
   @escape_cosmo: escape_cosmo
   @unescape_cosmo: unescape_cosmo