feat(render): per-flavour escaping and RFC 5545 line folding

Six flavours: latex, groff, html, xml, ics, none. Markdown, AsciiDoc and
plain text map to none deliberately -- their metacharacters are
context-dependent and escaping them aggressively produces worse output
than not escaping.

An unrecognised extension returns None rather than falling back to none:
guessing the flavour wrong produces malformed output that looks fine
until it does not.

Folding backs off to a non-continuation byte, so a fold never splits a
UTF-8 sequence -- the failure mode that would corrupt Polish and Latin
names in a published feed.

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