Inline `\ ` round-trip splits one `Str` into three inlines

[rundel]

A literal backslash followed by a space in inline text parses as a single Str containing the backslash and what follows. The writer emits the backslash as \\, which on re-parse produces a separate Str "\\" plus a Space plus the next Str. Source rendering is equivalent but the AST structure differs.

$ printf -- 'a \\ b\n'
a \ b

$ printf -- 'a \\ b\n' | cargo run --bin pampa --
[ Para [Str "a", Space, Str "\\ b"] ]

$ printf -- 'a \\ b\n' | cargo run --bin pampa -- -t qmd
a \\ b

$ printf -- 'a \\ b\n' | cargo run --bin pampa -- -t qmd | cargo run --bin pampa --
[ Para [Str "a", Space, Str "\\", Space, Str "b"] ]

A \ not followed by a space (e.g. foo\bar, or trailing \ for LineBreak in a paragraph) round-trips correctly. The shape that triggers this in the wild is a trailing \ inside a pipe-table cell (cell padding inserts the space):

• docs/authoring/article-layout.qmd#L351
• docs/authoring/front-matter.qmd#L107
• docs/blog/posts/2023-12-05-asa-traveling-courses/index.qmd#L21