Add ability for keywords to point to each other

[koubaa]

For instance a section could refer to a material - it would be good to have an API for the section keyword to get the material keyword

[koubaa]

This was implemented in #735 for tranid pointing to *DEFINE_TRANSFORM. It's usage could be expanded to other keywords.

[koubaa]

Plan:

Implement a get_links() API on keywords to traverse references at runtime, starting with one high-value link type (DEFINE_CURVE) and taking it through full API completion before expanding to other link types.

Steps

1. Add LinkType enum and base get_links() method to src/ansys/dyna/core/keywords/keyword.py: Create LinkType enum (starting with ALL, DEFINE_CURVE), and implement get_links(link_type: LinkType = LinkType.ALL) -> List[KeywordLink] that returns referenced keywords by querying the parent deck. KeywordLink design is open for now.

2. Create complete link ID mapping in codegen/keyword_generation/spec_loader.py: Add DEFINE_CURVE = 19 to LinkIdentity class with a reverse mapping dict, enabling the code generator to identify DEFINE_CURVE reference fields from kwd.json.

3. Extend the links generation handler in codegen/keyword_generation/handlers/links.py: Generalize to process DEFINE_CURVE (link=19) fields in addition to DEFINE_TRANSFORMATION (link=40), generating *_link properties (e.g., lcid → lcid_link) that return the referenced keyword.

4. Update Jinja template in codegen/templates/keyword.py.jinja2: Add _link_fields class attribute mapping field names to their link types, enabling get_links() to know which fields to traverse for each link type.

5. Regenerate keyword classes and add tests: Run python codegen/generate.py, then add tests in a new tests/test_keyword_links.py verifying: (a) lcid_link property returns correct DEFINE_CURVE, (b) get_links(LinkType.DEFINE_CURVE) returns all referenced curves, (c) get_links() returns all links.

6. Doc: Document the get_links() API and *_link properties in the keywords module docstrings, with usage examples showing how to traverse from a keyword (e.g., MAT_PIECEWISE_LINEAR_PLASTICITY) to its referenced DEFINE_CURVE.

Start with MAT_PIECEWISE_LINEAR_PLASTICITY since it has two fields that point to DEFINE_CURVE