fix(map): emit clear compile_error for invalid expr and nested-meta parse failures

[RAprogramm]

Problem

#[map(...)] attribute parsing in `crates/entity-derive-impl/src/entity/parse/field/map.rs` silently swallows errors, producing cryptic downstream failures instead of clear messages at the macro call site.

Two specific points

1. MapConfig::generate for Expr (lines 220–222):

```rust
Self::Expr(expr) => {
let expr_tokens: TokenStream = expr.parse().unwrap_or_default();
quote! { #expr_tokens }
}
```

If the user writes `#[map(expr = "invalid $$$ rust")]`, parse() fails, unwrap_or_default() returns an empty TokenStream, and the generated code references nothing. The compiler then complains about whatever surrounding expression was supposed to receive that value — far from the actual #[map(...)] site. The user has no clue the problem is in their attribute.

2. MapConfig::from_attr nested meta (lines 138–151):

```rust
let _ = meta_list.parse_nested_meta(|meta| {
if meta.path.is_ident("empty_to_none") { ... }
else if meta.path.is_ident("unwrap_default") { ... }
...
Ok(())
});
```

Errors from parse_nested_meta are bound to _ and dropped. If a nested item fails to parse (e.g. malformed expr =), the user sees MapConfig::None silently substituted.

Fix

• Replace unwrap_or_default() in generate with explicit compile_error! token emission that includes the original parse error message and the user-supplied expr string.
• Propagate parse_nested_meta errors instead of discarding them — surface them at the proc-macro level so they appear at the #[map(...)] span.

Tests

Add cases under crates/entity-derive/tests/cases/fail/:

• map_expr_invalid.rs — #[map(expr = "garbage $$$")] must produce a clear compile error pointing at the attribute, mentioning expr.
• map_nested_meta_invalid.rs — malformed nested item must produce a parse error at the attribute, not silently degrade to None.

Both verified via trybuild.