Add linearbearingblock model contract with folder-local registration

[seveibar]

Adds a generic four-hole linear-bearing block contract with explicit housing, shaft and cartridge dimensions. The independent nominal cartridge contract avoids a dependency on another unmerged family.

Example: linearbearingblock_bore8mm_bearingod15mm_w34mm_l24mm_h24mm_mount(clearance)_hole4.5mm_pitchx24mm_pitchy16mm.

Schema, strict parser, public exports and typed registration live in src/models/linearbearingblock/; no shared registry or export file changes. Documentation distinguishes nominal visual internals from the overall dimensional envelope.

Schema, strict parser, public schema/type exports and typed registration remain in each src/models/<name>/ folder; generated discovery owns shared unions and barrels.

linear bearing block

Paired renderer: tscircuit/jscad-electronics#441. Direct full-string fixture; verified browser preview.

linearbearingblock_bore8mm_bearingod15mm_w34mm_l24mm_h24mm_mount(clearance)_hole4.5mm_pitchx24mm_pitchy16mm

The canonical housing is 34×24×24 mm, with 4.5 mm clearance holes on a 24×16 mm pattern. Its mounting face is Z=0; the shaft travels along Y at X=0 and Z=height/2. Housing dimensions and nominal cartridge internals are independently specified generic geometry, without manufacturer-interchangeability, preload or fit claims.

Contract tests, typecheck, formatting, build and diff checks passed locally. Source-group result: 83 pass, 0 fail, 2400 assertions.

Four-view snapshots (isometric, top, front and side):

Contract test workflow passed in CI at this exact head. Other passing checks: preview.

The combined contract union passed tests (105 tests, 0 failures, 2,994 assertions), typecheck, formatting and build.

Manual review only; auto-merge is not enabled. Geometry remains in the paired renderer PRs.