Provide a parametric combinator that is atLeast but preserves owned configs

[vic]

This introduces parametric.withOwn that is just a parametric.atLeast that also includes the aspect owned configs.

See discussion: #91 (reply in thread)

Calling parametric x itself is an alias for parametric.withOwn x.

So that both syntax do the same:

den.aspects.foo = {
  __functor = den.lib.parametric;
  nixos.foo = 1;
  includes = [ bar ];
};

and also, *this is the preferred syntax that will be documented, since it is more intuitive and does not surfaces __functor:

den.aspects.foo = den.lib.parametric {
  nixos.foo = 1;
  includes = [ bar ];
};

Still have to update documentation about this combinator.

Fixes #97.

[Copilot]

Pull Request Overview

This PR introduces a new parametric.withOwn combinator that combines parametric aspect behavior with owned configurations, providing a more intuitive API for defining aspects. The preferred syntax den.lib.parametric { ... } now creates aspects that include both their static configs and parametric includes.

Key changes:

• Added parametric.withOwn function that combines owned and parametric.atLeast behaviors
• Updated parametric.__functor to dispatch to withOwn when called with attribute sets (previously dispatched to fixedTo)
• Renamed parameter from ctx to arg in __functor for clarity

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
nix/lib.nix	Adds parametric.withOwn combinator and updates __functor to make parametric { ... } an alias for withOwn, with parameter renaming for clarity
modules/aspects/dependencies.nix	Refactors to extract context into a named binding and explicitly uses parametric.fixedTo to preserve existing behavior after __functor changes
templates/examples/modules/_example/ci/parametric-with-owned.nix	Adds comprehensive test demonstrating the new parametric.withOwn functionality with context forwarding and owned config inclusion

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