chore: scope subscript/superscript delimiters as punctuation.definition (#21)

Follow-up polish from the #17 review. The ~ / ^ delimiters now get their
own punctuation.definition.{subscript,superscript}.{begin,end}.rxiv scopes
via captures, consistent with the existing markup.bold / markup.italic
rules, so themes can style the delimiters independently of the content.

Match behaviour is unchanged: the regexes still match H~2~O, CO~2~,
E=mc^2^, x^n^ and still ignore ~~strikethrough~~ and [^1] footnotes. Folds
into the unreleased 0.3.17 entry (no version bump).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: paxcalpt

CHANGELOG.md

@@ -7,7 +7,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/), and this
 ## [0.3.17] - 2026-05-29
 
 ### Fixed
-- **🔧 Subscript/superscript scope leak** - The token colour rule targeted the generic `markup.subscript` / `markup.superscript` TextMate scopes, which other grammars (standard Markdown, AsciiDoc, reStructuredText) also emit. Because `editor.tokenColorCustomizations` is contributed globally, this greyed out subscript/superscript text in every file type, not just rxiv documents. The scopes are now namespaced to `markup.subscript.rxiv` / `markup.superscript.rxiv`, and the grammar gained matching rules for `~subscript~` and `^superscript^`, so the colouring is both scoped to rxiv files and actually applied (the previous rule matched nothing the grammar emitted).
+- **🔧 Subscript/superscript scope leak** - The token colour rule targeted the generic `markup.subscript` / `markup.superscript` TextMate scopes, which other grammars (standard Markdown, AsciiDoc, reStructuredText) also emit. Because `editor.tokenColorCustomizations` is contributed globally, this greyed out subscript/superscript text in every file type, not just rxiv documents. The scopes are now namespaced to `markup.subscript.rxiv` / `markup.superscript.rxiv`, and the grammar gained matching rules for `~subscript~` and `^superscript^`, so the colouring is both scoped to rxiv files and actually applied (the previous rule matched nothing the grammar emitted). The `~`/`^` delimiters are also scoped as `punctuation.definition.{subscript,superscript}.{begin,end}.rxiv`, consistent with the bold/italic rules, so themes can style the delimiters independently of the content.
 
 ## [0.3.16] - 2026-05-29
 

syntaxes/rxiv-markdown.tmLanguage.json

@@ -166,12 +166,20 @@
     },
     {
       "name": "markup.subscript.rxiv",
-      "match": "(?<!~)~([^~\\s]+)~(?!~)",
+      "match": "(?<!~)(~)([^~\\s]+)(~)(?!~)",
+      "captures": {
+        "1": {"name": "punctuation.definition.subscript.begin.rxiv"},
+        "3": {"name": "punctuation.definition.subscript.end.rxiv"}
+      },
       "comment": "Subscript text with ~text~ (e.g. H~2~O); single tilde, no spaces, avoids ~~strikethrough~~"
     },
     {
       "name": "markup.superscript.rxiv",
-      "match": "\\^([^\\^\\s]+)\\^",
+      "match": "(\\^)([^\\^\\s]+)(\\^)",
+      "captures": {
+        "1": {"name": "punctuation.definition.superscript.begin.rxiv"},
+        "3": {"name": "punctuation.definition.superscript.end.rxiv"}
+      },
       "comment": "Superscript text with ^text^ (e.g. E=mc^2^)"
     },
     {