Per the Python spec, the whitespace up to the first backslash determines
the line's indentation, and continuation lines should not affect the
indentation level. The lexer's `eat_indentation` loop was not handling
this: after a `\` continuation, it accumulated the continuation line's
whitespace into the indentation, causing spurious IndentationError and
"Expected a statement" errors.

Fix: after consuming `\<newline>` during indentation tracking, skip the
continuation line's whitespace with `eat_while(is_python_whitespace)`
without accumulating it into `indentation`. Guard: only skip when
`indentation != Indentation::root()` — when `\` is at column 0, let the
loop continue so the next line's whitespace is accumulated normally.

Lexer snapshot tests cover: basic continuation in indented block,
continuation at column 0, multiple consecutive continuations, and
mismatched indentation error — each with all three EOL types.

Ref: https://docs.python.org/3/reference/lexical_analysis.html#indentation
See: python/cpython#90249