Clarify or decouple GableRoof axis from long-span bridge height

Production finding

MinePilot's coverage-batch review found that a long covered bridge cannot
express the expected transverse gable roof without a structural axis rewrite.
This is a CraftDAG API/expressiveness issue, not a MinePilot deterministic
review bug.

Minimal shape

A bridge plan used a long RoomShell:

{
  "placement": {
    "anchor": { "x": 1, "y": 4, "z": 2 },
    "size": { "width": 43, "height": 4, "length": 7 }
  }
}

With GableRoof { over: "covered_span", direction: "x", overhang: 1 },
the plan compiles within a 45 × 14 × 11 bound, but MinePilot's live 3D review
reads as long parallel roof/rail strips rather than a conventional covered
bridge roof.

Changing only the roof direction to "z" gives the visually expected
transverse orientation, but compilation fails:

GableRoof "gable_cover" exceeds ComponentPlan height bounds.

The height calculation treats the 43-block long span as the roof-height-driving
span. The only current workaround is to rewrite the whole bridge around the
other axis, including entry walls, window walls, deck, truss/rail components,
and metadata dimensions.

Why this matters

For a normal coverage candidate, that is not a low-risk targeted repair. It
forces authors either to publish an implausible roof silhouette or abandon a
useful, common structure family. Similar RoomShell-only compositions also make
natural mine entrances read as rectangular boxes, but this issue intentionally
focuses on the deterministic GableRoof axis contract.

Requested investigation

1. Document the exact GableRoof.direction semantics and height calculation.
2. Determine whether a gable can specify ridge/slope axis independently from
   the authored room's width/length orientation.
3. If current behavior is intentional, expose a clearer orientation option or
   provide a recommended bridge-oriented helper/primitive.
4. Add a small regression fixture for a 43 × 7 covered bridge asserting the
   intended transverse gable stays in bounds and compiles.

No change to MinePilot or CraftDAG is proposed by this report; this is a
production reproduction and API/expressiveness investigation request.

Start by tracing the GableRoof direction semantics and height calculation in the ComponentPlan path, using the 43 × 7 RoomShell reproduction as the entry point. Determine whether ridge and slope axes can be decoupled from authored width and length; done means the contract is documented or clarified and a regression fixture confirms the intended transverse gable compiles within bounds.