fix(quad): bound corner silkscreen arms to prevent pad overlap on narrow variants

[rudra496]

Summary

Fixes #734.

Previously, corner silkscreen paths on quad packages were assigned a fixed arm length (`csz = pw * 2`) from `(±w/2, ±h/2)`. On narrow-body variants (such as `tqfp32_w7` and `tqfp48_w7`), the inward arms crossed through the outermost pad copper, resulting in negative silk-to-pad clearance.

Changes

1. Dynamic Corner Arm Bounding: Computed distance from the corner origin to outermost pad bounds along both horizontal and vertical axes (`availX`, `availY`), capping `csz` to maintain IPC silk-to-pad clearance in `src/fn/quad.ts`.
2. Unit Tests: Added `tests/tqfp-narrow-body-silkscreen.test.ts` verifying that corner silkscreen arms on `tqfp32_w7` do not intersect outermost pads.

Verification

• `bun test tests/tqfp-narrow-body-silkscreen.test.ts` (1/1 passing).

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