Add gremlin/io package export exposing GraphBinary serializers

[danielbodart]

Adds a gremlin/io package export exposing the GraphBinary type serializers.

They are currently only reachable by importing past the exports map into build/, which couples consumers to internal file layout and breaks under bundlers that honour exports (e.g. esbuild/wrangler). This lets server-side v4 protocol implementations reuse them cleanly:

import ioc from 'gremlin/io';

Follows the ./language subpath precedent (import + types). CHANGELOG updated. Happy to rename the subpath or add a CJS require entry if you'd prefer.

Not tied to a JIRA yet (glad to file one). Scope is the export map + changelog only; not run through the full Maven build locally. Drafted with AI assistance (Assisted-by trailer on the commit).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.42%. Comparing base (a28cd1f) to head (639df0b).
⚠️ Report is 236 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3511      +/-   ##
============================================
+ Coverage     76.35%   76.42%   +0.06%     
- Complexity    13424    13967     +543     
============================================
  Files          1012     1031      +19     
  Lines         60341    63139    +2798     
  Branches       7075     7389     +314     
============================================
+ Hits          46076    48255    +2179     
- Misses        11548    11877     +329     
- Partials       2717     3007     +290     

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