Consolidate the Write Guard's Shell Context Scanners Into One Walker #3354

Context

A design finding from the local review of the backslash-newline narrowing (branch feature/guard-backslash-newline-quoting), declined there because consolidating means changing shared helpers on every tokenizer path of a security hook inside a narrowing change.

Finding

gh-write-guard.py holds several hand-written scanners of bash's quoting and nesting contexts, and the narrowing adds one more, _line_break_contexts. Its inner readers repeat existing ones:

• its double-quote reader repeats _skip_quote_body
• its command reader repeats _skip_group with comments=True: paren depth, case counting, a # at a word start, and pending heredocs from _HEREDOC_IN_GROUP
• its heredoc reader repeats _skip_heredoc_lines and _heredoc_body_end, and adds a rule those lack, that a line ending in an unescaped backslash in an unquoted heredoc joins the next, so the next is not its delimiter
• its quoted-delimiter test repeats the one in _unquoted_heredoc_bodies

Each copy is a place for the same bash rule to be read two ways, as the heredoc continuation rule already is.

Possible Direction

One context walker over _skip_group and _skip_quote_body, taking an optional visitor that records the context at each index it passes. The skippers keep their return contract, _line_break_contexts becomes a visitor, and the heredoc readers share one body-end rule.