Bug

When mdsh processes a file in-place that contains no mdsh tags, it corrupts the file on repeated runs by deleting lines from the end.

Reproduction

Build from source

nix develop --command cargo build --release

Case 1: File with trailing newline

Trailing newline stripped on first run, last line deleted on second run

nix develop --command bash -c 'printf "# No mdsh tags\n\ntext\n" > /tmp/test.md && ./target/release/mdsh --input /tmp/test.md && diff <(printf "# No mdsh tags\n\ntext\n") /tmp/test.md'

Run again — "text" is now gone

nix develop --command bash -c './target/release/mdsh --input /tmp/test.md && diff <(printf "# No mdsh tags\n\ntext\n") /tmp/test.md'

Case 2: File without trailing newline

Last line dropped on first run

nix develop --command bash -c 'printf "# No mdsh tags\n\ntext" > /tmp/test2.md && ./target/release/mdsh --input /tmp/test2.md && diff <(printf "# No mdsh tags\n\ntext") /tmp/test2.md'

Root cause

Two interacting issues in src/main.rs:

1. trim_ascii_end (applied on in-place writes) strips all trailing ASCII whitespace including \n, removing the file's trailing newline.
2. The parser's catch-all in src/parser.rs matches lines with (take_until("\n"), newline), which requires a trailing \n. Lines without one are silently dropped.

This creates a cascading failure: each run strips the trailing newline, causing the next run to drop the now-unterminated last line.

Expected behavior

Running mdsh on a file with no mdsh tags should leave it unchanged, regardless of whether it has a trailing newline.