Bevel gear placement: pitchbase / apex reference frames (replaces BOSL2 anchors)

[gsdali]

Part of #84. Blocks the docs page (#89).

Why this is needed

The epic explicitly declines to port BOSL2's attachment system (anchor / spin / orient /
reorient / named_anchor / attachable). It is thousands of lines of BOSL2 infrastructure
with no OCCT analogue.

But Examples 2 and 3 of the deliverable both depend on it. Example 2 meshes a gear pair
using anchor="apex", orient=FWD; Example 3 positions manually using pitch_radius() offsets
plus spin=180/t2. So we need the outputs of the anchor system without the machinery.

What to build

BOSL2 computes exactly three named anchors (gears.scad:2642-2646):

named_anchor("pitchbase", [0,0,pitchoff-ctr_thickness/2+backing/2]),
named_anchor("flattop",   [0,0,ctr_thickness/2+backing/2]),
named_anchor("apex",      [0,0,hyp_ang_to_opp(pitch_angle<90?ocone_rad:icone_rad,90-pitch_angle)
                              +pitchoff-ctr_thickness/2+backing/2])

Expose these as three reference Z heights on the returned gear: a small result struct
alongside the Shape, surfaced into the manifest metadata so render-preview and downstream
consumers can use them. Callers then position with ordinary translated and rotated.

Why the apex matters

For a bevel pair to mesh, the pitch cone apexes must coincide (gears.scad:2363). Anchoring
both gears at their apex and rotating one by the shaft angle is the whole trick. It is why
BOSL2's Example 2 is three lines. Getting this right is what makes a meshing pair demonstrable
rather than eyeballed.

Worth capturing as a helper: given two gears and a shaft angle, return the transform that
meshes them. That is the piece a downstream consumer actually wants, and the correct
replacement for anchor="apex".

Acceptance criteria

• pitchbase, flattop and apex heights computed and exposed per gear
• A meshing helper positions a pair at an arbitrary shaft angle with coincident apexes
• Verified at 90 degrees (Examples 1-3) and at least one non-right angle
• Apex coincidence verified numerically, not just visually. Assert the two computed apex
  points agree within tolerance
• Decision recorded as an OKF entry: explicit reference frames replace BOSL2 anchors