Challenge 3 — Solution

Task: Explain why course_name_index.md exists as its own separate
file rather than being folded into course_folder_mapping.md, given
that both files map a prompt prefix to information about a course.

The two files map a prompt prefix to two genuinely different kinds of
information, used at two different times for two different purposes,
which is exactly why keeping them separate makes sense despite the
surface-level overlap.

course_folder_mapping.md answers "where does this course's content
actually live on disk?" - it's consulted while GENERATING content, to
determine the real folder path a new chapter should be saved to. Its
own entries are necessarily detailed: full folder paths, notes about
real on-disk casing quirks, status notes about completeness.

course_name_index.md answers a narrower, different question: "what is
this course's real, human-readable full name?" - it exists purely to
support P11's own rule (Chapter 3), which requires reader-facing prose
inside a chapter to reference other courses by their real name ("PHP
Intermediate 4"), never by the bare internal prompt shorthand
("php2-4"). It's consulted while WRITING a chapter's own body text, a
completely different moment and purpose than folder-path lookup.

Keeping them separate means course_name_index.md can stay genuinely
flat and fast to scan - just prefix and full name, nothing else -
without being cluttered by the folder-path detail and status notes
that course_folder_mapping.md legitimately needs for its own different
job. Merging them would mean every quick "what's the real name for
seo1?" lookup has to wade through folder-path and casing information
that's irrelevant to that specific question.

Notes:
- Both files are still kept in sync with each other in practice - the
  chapter notes that if a prefix isn't yet in course_name_index.md, the
  missing entry gets added by looking it up in course_folder_mapping.md
  (or completed_courses.md) in the same step, rather than the two files
  being allowed to drift apart from each other.
- This is a good small example of a more general principle worth
  noticing across this whole rules system: splitting a file by WHO
  reads it and WHY, rather than merging everything that happens to
  reference the same prompt prefix into one large file.