Refuse case-folding collisions when scaffolding blueprint paths

[Deicyde]

On a case-insensitive filesystem, a project may legitimately contain a Lean
library directory named Blueprint/. scaffold_project currently checks and
writes lowercase managed paths through ordinary Path.exists()/path joins, so
blueprint/roadmap can resolve into that unrelated Lean library.

PR #14 correctly makes read-only project inspection recognize optional
Autoform scaffold paths only at their exact portable spellings. The write path
needs its own fail-closed collision check; making inspection reject every
near-case name caused real formal-math inspection to fail.

Acceptance criteria:

• Before writing a managed scaffold path, enumerate the exact parent directory
  entry and reject any distinct casefold/NFC collision.
• Cover at least blueprint, mkdocs.yml, .github, workflows, and the two
  managed workflow filenames.
• Return a clear ScaffoldError and leave the project byte-for-byte unchanged.
• Add a platform-independent regression harness plus a real case-insensitive
  filesystem test when available.