atdml: support <ocaml attr=...> on record fields, variant constructors, and payloads

[pedrobslisboa]

Add support for the ocaml attr annotation in three new positions in atdml: record fields, variant constructors, and variant payload types.

Type-definition-level attr ([@@attr]) was already supported; this extends it to the three remaining ATD positions.

Test plan:

• New variant attr snapshot test covering constructor-level and payload-level attr
• Existing attr snapshot test updated with a field-level attr example
• Docs updated in doc/atdml-reference.rst and CHANGES.md

🤖 Generated with Claude Code

[mjambon]

Looks good to me, thanks! I fixed the merge conflict on the changelog.

[mjambon]

dlang.org is down, giving us an error on these lines:

# To get latest ldc version
source $(curl https://dlang.org/install.sh | bash -s ldc -a)
echo "source $(~/dlang/install.sh ldc -a)" >> "$BASH_ENV"

Looking into using the ubuntu package instead (#499)

[mjambon]

Hmm. I'll let you rebase against the master branch. It should solve the source: filename argument required error.