fix(id): output into two files when structs present

[jdacoello]

This PR addresses an issue introduced by #528

Problem

vspec export id writes static UIDs (staticUID) for every node in the tree into a single flat, FQN-keyed YAML dict, which is meant to be re-loaded as a normal vspec/overlay file downstream.

When --types was supplied, the struct/type tree's nodes were merged into that same dict as the main tree's nodes. Since both the main tree's root (e.g. Vehicle) and the types tree's root (e.g. Types) are separator-less top-level keys, the generated file ended up with two roots. Feeding that file back into any other vss-tools command (as --overlays) then failed with MultipleRootsException, since a single vspec file is only allowed to have one root.

Root cause

In id.py's cli(), both trees were exported into the same dict:

id_counter, _ = export_node(signals_yaml_dict, tree, id_counter, case_sensitive)
if datatype_tree:
    id_counter, _ = export_node(signals_yaml_dict, datatype_tree, id_counter, case_sensitive)

Other exporters (json, yaml, csv, binary) already had a --types-output option to write type/struct data to a separate file — id.py never wired it up, even though id.md already (incorrectly) documented it as if it existed.

Fix

• Struct/type static UIDs are now written to a separate dict/file instead of being merged into the main output.
• Added --types-output to vspec export id, consistent with other exporters.
• --types-output is optional: if --types is used and --types-output is not given, the tool now derives a default name automatically: structs_<output-filename> written next to --output (e.g. --output out.vspec → structs_out.vspec).
• Updated CHANGELOG.md and id.md (options table + new "Custom data types (structs)" section) to document the new behavior and the recommended chaining workflow (main output as -l/--overlays, types output as -t/--types — replacing, not supplementing, the original types file).
• Updated/added tests in test_id_struct.py covering both explicit --types-output and the new default-naming fallback.

Usage change

# Before (broken): struct UIDs merged into the main output -> two roots, unusable downstream
vspec export id -s spec.vspec --types types.vspec -o ids_overlay.vspec

# After: struct UIDs written separately (explicit name)
vspec export id -s spec.vspec --types types.vspec \
  -o ids_overlay.vspec --types-output ids_types.vspec

# After: struct UIDs written separately (default name: structs_ids_overlay.vspec)
vspec export id -s spec.vspec --types types.vspec -o ids_overlay.vspec

# Chaining downstream:
vspec export <format> -s spec.vspec -l ids_overlay.vspec --types ids_types.vspec ...

Testing

• test_id_struct.py: verifies struct/property nodes land in the types-output file (not the main one) both when --types-output is explicit and when it's omitted (default structs_ naming).
• Full id/struct-related test suite passes (157 passed); 2 pre-existing, unrelated protobuf test failures confirmed to exist on main as well.