build: make Mldoc cross-buildable with Dune

[tiensonqin]

Summary

• wrap the Mldoc library so downstream native builds can depend on a stable module namespace
• export Nested_link and Pos through the Mldoc entry module
• pin the merged Dune-enabled Logseq XMLM package
• modernize CI for the declared OCaml 4.14 minimum and OCaml 5.x
• make formatting CI read-only instead of auto-committing to PR branches

Validation

• dune build --root . @install
• workflow YAML parse
• parent logseq-chat iOS arm64 cross-compilation through Dune

Dependency

The XMLM dependency was merged to logseq/xmlm master at f6c11f85125da62e6b97f3947e91d6615653c2c3.

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