refactor: extract pure file collection logic for testability

- Split collect_input_files into pure collect_from_descriptors + I/O wrapper
- Replace 2 tempfile-based tests with 5 pure unit tests
- Add unit tests for --dry-run, --force, --quiet, --heading-offset flags
- Remove tempfile dev-dependency

Test coverage improved without filesystem access requirements.

Author: whee

Cargo.lock

@@ -8,7 +8,7 @@ name         = "cp2md"
 readme       = "README.md"
 repository   = "https://github.com/whee/cp2md"
 rust-version = "1.91"
-version      = "1.4.0"
+version      = "1.4.1"
 
 [dependencies]
 chrono     = { version = "0.4", features = ["serde"] }
@@ -18,9 +18,6 @@ serde_json = "1"
 snafu      = "0.8"
 walkdir    = "2"
 
-[dev-dependencies]
-tempfile = "3"
-
 [lints.rust]
 unsafe_code = "forbid"