feat(render): Typst as a seventh escaping flavour

Adds Escape.Typst: type, all, to_string ("typst"), of_string,
of_extension (.typ), and a per-character expand function, structured
identically to the existing latex escaper (one pass, no re-scan, so
double-escaping stays impossible by construction).

The metacharacter set was verified against the installed typst 0.14.2
binary, not assumed: a probe document escaping each of #, *, _, $, @,
<, >, `, \, ~ and - was compiled and pdftotext'd back to confirm the
literal character survives, and each was separately confirmed to do
something else when left bare (# opens code mode, */_ toggle
strong/emph, $ opens math, @ opens a reference -- a bare unresolved
@word is a hard compile error, not merely mangled output -- </> can
close around a bare word into label syntax that swallows it whole, `
opens raw, ~ is a non-breaking space, and a run of two or three '-'
becomes an en/em dash). All ten are backslash-escapable; none needed a
non-backslash workaround. '-' is escaped unconditionally rather than
only inside a detected run, since this escaper has no lookahead -- a
probe confirmed escaping every hyphen independently still typesets as
literal hyphens for a run of any length, so the single per-character
rule is sufficient.

test_escape.ml's new Typst cases were written first and shown to fail
against a stubbed identity apply before the real escaper landed, per
this project's own regression-test discipline. Comments in both files
mark which new tests are genuine regression tests (the per-character
escaping, including a real shipped citation and a synthetic
markdown-habit overlay name) versus characterisation (to_string/
of_string/of_extension are flat table lookups with no logic to have
been wrong).

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