ADFA-4128: name resDirs among the layout's disk reads

Akash on #1719: the class KDoc exists to tell a caller which members are
expensive, and it left out resDirs(), whose isDirectory filter stats the
disk. A caller reading "the path accessors are arithmetic and cost nothing"
would have counted resDirs() among them.

It stats rather than walks, so it is cheaper than the three already named;
the sentence says that rather than lumping it in with them.

Comment only. No behaviour change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XN91S41dghWHoXAoWhJAnm

Author: fryanpan

quickbuild/core/src/main/java/org/appdevforall/cotg/quickbuild/data/QuickBuildProjectLayout.kt

@@ -12,7 +12,8 @@ import java.io.File
  *
  * Not all of it is arithmetic: [allSources], [watchedRoots] and [watchedFiles] walk the tree
  * (the latter two through the module scan), so they are disk reads and belong off the main
- * thread. The path accessors are arithmetic and cost nothing.
+ * thread. [resDirs] only stats rather than walks, but that is still a disk read. The remaining
+ * path accessors are arithmetic and cost nothing.
  *
  * @property projectRoot the user project's root directory, which the watched gradle config
  *   files and the module scan hang off.