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).

2 files changed, 34 insertions, 9 deletions

diff --git a/lib/render/escape.ml b/lib/render/escape.ml
index f506c44..ee9ce6c 100644
--- a/lib/render/escape.ml
+++ b/lib/render/escape.ml
@@ -1,18 +1,19 @@
-type flavour = Latex | Groff | Html | Xml | Ics | None_
+type flavour = Latex | Typst | Groff | Html | Xml | Ics | None_
 
-let all = [ Latex; Groff; Html; Xml; Ics; None_ ]
+let all = [ Latex; Typst; Groff; Html; Xml; Ics; None_ ]
 
 let to_string = function
-  | Latex -> "latex" | Groff -> "groff" | Html -> "html"
+  | Latex -> "latex" | Typst -> "typst" | Groff -> "groff" | Html -> "html"
   | Xml -> "xml" | Ics -> "ics" | None_ -> "none"
 
 let of_string = function
-  | "latex" -> Some Latex | "groff" -> Some Groff | "html" -> Some Html
+  | "latex" -> Some Latex | "typst" -> Some Typst | "groff" -> Some Groff | "html" -> Some Html
   | "xml" -> Some Xml | "ics" -> Some Ics | "none" -> Some None_
   | _ -> None
 
 let of_extension = function
   | ".tex" -> Some Latex
+  | ".typ" -> Some Typst
   | ".ms" | ".mom" | ".me" -> Some Groff
   | ".html" | ".htm" -> Some Html
   | ".xml" -> Some Xml
@@ -37,6 +38,29 @@ let latex = function
   | '~' -> "\\textasciitilde{}"
   | c -> String.make 1 c
 
+(* Typst markup metacharacters, verified live against the installed `typst`
+   0.14.2 binary rather than assumed (probe: each character escaped,
+   compiled, pdftotext'd back, confirmed literal; each also confirmed to
+   do something ELSE when left bare -- '#' opens code mode, '*'/'_' toggle
+   strong/emph, '$' opens math, '@' opens a reference (a bare "@word" is a
+   hard COMPILE ERROR, not merely mangled output, if "word" resolves to no
+   label), '<'/'>' can close around a bare word into label syntax that
+   swallows it whole, '`' opens raw, '~' is a non-breaking space, and a run
+   of two or three '-' becomes an en/em dash). Backslash-escapable in every
+   case -- no character on this list needed a non-backslash workaround.
+   '-' is escaped UNCONDITIONALLY, not only inside a detected run: this
+   function has no lookahead (see [expand]'s own comment above), so it
+   cannot tell "is this hyphen part of a run" one character at a time: a
+   probe confirmed escaping every hyphen independently ("\-\-\-") still
+   typesets as three literal hyphens, never collapsing to an em dash, so
+   the single per-character rule is sufficient for runs of any length. *)
+let typst = function
+  | '\\' -> "\\\\"
+  | '#' -> "\\#" | '*' -> "\\*" | '_' -> "\\_" | '$' -> "\\$"
+  | '@' -> "\\@" | '<' -> "\\<" | '>' -> "\\>" | '`' -> "\\`"
+  | '~' -> "\\~" | '-' -> "\\-"
+  | c -> String.make 1 c
+
 let html = function
   | '&' -> "&amp;" | '<' -> "&lt;" | '>' -> "&gt;"
   | '"' -> "&quot;" | '\'' -> "&#39;"
@@ -57,6 +81,7 @@ let groff s =
 let apply flavour s =
   match flavour with
   | Latex -> expand latex s
+  | Typst -> expand typst s
   | Groff -> groff s
   | Html | Xml -> expand html s
   | Ics -> expand ics s
diff --git a/lib/render/escape.mli b/lib/render/escape.mli
index 97cf050..ee1e20b 100644
--- a/lib/render/escape.mli
+++ b/lib/render/escape.mli
@@ -3,11 +3,11 @@
     Knows nothing about calendars. Pure and total: every function is defined on
     every string, and none reads the clock, the environment or the filesystem. *)
 
-(** The six escaping modes. Markdown, AsciiDoc and plain text all use [None_]:
-    their metacharacter sets are context-dependent, and escaping them
-    aggressively produces worse output than not escaping at all (spec section 5).
-    This is a documented limitation of those flavours. *)
-type flavour = Latex | Groff | Html | Xml | Ics | None_
+(** The seven escaping modes. Markdown, AsciiDoc and plain text all use
+    [None_]: their metacharacter sets are context-dependent, and escaping
+    them aggressively produces worse output than not escaping at all (spec
+    section 5). This is a documented limitation of those flavours. *)
+type flavour = Latex | Typst | Groff | Html | Xml | Ics | None_
 
 val all : flavour list
 val to_string : flavour -> string