Add linearballbearing model contract with folder-local registration

[seveibar]

Adds generic recirculating linear ball-bearing contracts, with common de-facto sleeve envelopes for 8, 10 and 12 mm shafts. The string specifies dimensions and uses no supplier series identifiers.

Example: linearballbearing_bore8mm_od15mm_l24mm_seals(both).

Schema, strict parser, public exports and typed registration live in src/models/linearballbearing/; 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 ball bearing

Paired renderer: tscircuit/jscad-electronics#440. Direct full-string fixture; verified browser preview.

linearballbearing_bore8mm_od15mm_l24mm_seals(both)

The canonical sleeve is 8 mm bore, 15 mm outside diameter and 24 mm long. Travel is along +Z between terminal planes Z=0 and Z=length. Loaded rows, offset return rows and relieved end chambers form a nominal recirculating assembly; these chosen internal dimensions do not claim supplier geometry, ISO 10285 fits, preload or load ratings.

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.