docs: VAJAX_CACHE_DIR env var documented but not implemented

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Problem

Both docs/configuration.md (line 58) and docs/cli_reference.md (line 183) document a `VAJAX_CACHE_DIR` environment variable that supposedly overrides the cache directory:

`VAJAX_CACHE_DIR` — Override cache directory (default: `~/.cache/vajax/`)

However, this environment variable is not implemented anywhere in the codebase. A search across all `vajax/` Python files finds zero references to `VAJAX_CACHE_DIR`.

Actual behavior

The cache directory is determined by the XDG Base Directory Specification via `XDG_CACHE_HOME` (see `vajax/init.py:_get_xdg_cache_dir()`). The only way to override the cache directory is:

• Set `XDG_CACHE_HOME` (standard XDG variable)
• Call `vajax.configure_xla_cache(cache_dir=...)` programmatically
• Set `JAX_COMPILATION_CACHE_DIR` for the XLA part

Fix options

Either:

1. Implement `VAJAX_CACHE_DIR` in `_get_xdg_cache_dir()` / `configure_xla_cache()`, or
2. Remove the documentation and point users to `XDG_CACHE_HOME` and `JAX_COMPILATION_CACHE_DIR`