Model stackable joinery frames with removable Altra cassettes

[briansrls]

Superseded: Superseded by the integrated-corner cassette direction in #13295. The extra fixed-frame geometry is intentionally not incorporated; the branch is retained as historical reference.

The cassette previously depended on a fixed 2×2 rack. This adds a stackable outer frame while keeping each cassette independently removable from the front. The review opens with two units stacked and the lower cassette withdrawn 150 mm.

All 17 r06 cassette geometries and STL files remain byte-identical, preserving the parts already printing. Each unit adds 13 frame pieces (six side sections, six cross-tie halves, one lift-out stop) and two clip-on front skids. Lowered runners clear the existing rear sockets; the front cross-tie clears the skids during withdrawal. The frame uses integral locating feet/pockets and keyed joints, with no additional screws proposed. Shared power is reserved alongside the stack.

Geometry, placements and travel are modeled in DAG and exported through the existing CadQuery realization. The private review site retains individual rotation and downloads, adding a two-level assembly STEP.

Validation:

• Four stacking witnesses and all six existing cassette witnesses passed.
• CadQuery 2.8.0: all 32 printed instances / 25 authored geometries valid, connected, within the 180 mm envelope; STL manifoldness/bounds/volume and STEP round-trip passed.
• Zero printed-part intersections within one seated unit or between two seated levels.
• Seven withdrawal positions over 0–150 mm passed with the stop removed; this is sampled interference checking, not a continuous swept-volume or cable proof.
• Browser checks passed for stack/stop/extraction, keyboard rotation, roster, downloads, interfaces and mobile layout.

Review limitations: no stack load rating, populated-height limit, positive joint-retention qualification, creep/racking test or assembly-time measurement. Board supports and electrical/thermal design remain unresolved. Kernel-observation judgments remain in the existing Python packaging adapter; source witnesses do not close that existing review obligation. No new print is started by this change.

Stacked on #13222 to keep this revision separate while the original cassette PR is reviewed. Hosted CI has not yet been verified for this new branch.

Review: https://altra-chassis-workbench.briansrls448156.chatgpt.site