Add raw string literals (r'…') ahead of making triple quotes non-raw

[liamfd]

Transitional release. Adds r-prefixed raw string literals at every delimiter width — r'…', r"…", r'''…''', r"""…""" — so that backslash-carrying data has somewhere to live before the next release makes bare '''…''' and """…""" interpret the escape whitelist. After that, a bare triple-quoted string stops being a safe place for a backslash. This is the only window in which both spellings work.

Why. Triple quotes are carrying two jobs that one delimiter can't serve. They need to interpret escapes, so '''today\'s plan''' stops shipping its backslash to users. They also need to stay raw, because JSON and RUN parse their own escapes and any escape the tokenizer eats is one the inner layer never sees:

'''{"a": "x\ny"}''' JSON>   ->  Bad control character in string literal
'''{"p": "C:\\Users"}''' JSON>  ->  Bad escaped character in JSON
""" 'a\\nb' """ RUN         ->  escapes processed twice

Both sit on paths docs/forthic-prompt.md documents. Splitting the jobs across two spellings serves both; narrowing the whitelist would serve neither fully.

Structure. is_raw_string_start() plus one branch in transition_from_START, mirroring the existing <<'''…''' marked-redirect lookahead, and a raw flag on transition_from_GATHER_STRING. The triple-quote gather does no escape processing, so the triple-width forms reuse it unchanged and are aliases today. The prompt presents r as an escape hatch for content whose escapes something else has to read — not as a blanket "contains a backslash" rule — and warns that the bare triple form won't keep a backslash.

Compatibility. One narrow break, not a pure addition: words are gathered without breaking on quote characters, so a token spelled r + quote (r'don't') used to lex as a single word and now opens a raw string. No standard-library word is spelled that way, and definition names can't contain quotes, so no definable word is affected. Hence the patch release.

Design. r is the strongest prior available (Python, Rust, Nim, D, Scala) and Forthic already borrowed Python's '''. Backticks were ruled out despite Go and JS — generated Forthic is routinely wrapped in markdown fences. Lowercase only; no composition with the redirect marker. Unlike Python, a raw Forthic string may end in a backslash (r'C:\' → C:\), though the same rule means it can't contain its own delimiter.

Testing. 1011 passing, plus build and smoke (including the self-verifying prompt-examples check). Covers the JSON/RUN/path idioms end to end, greedy quote-runs with the prefix, mid-word collision safety, declined redirect composition, empty and unterminated raw strings, and an r-string streamed across every chunk boundary. Disabling the triple-width dispatch fails 9 tests.

Deferred to the breaking release, where the escape flip makes them testable: a streaming chunk boundary landing on a backslash collapses the cumulative value instead of growing it, and StringRedirectRouter.feed() diffs by length alone — so the sink and the stack would silently disagree.

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