Add board viaPlugging process selection

[techmannih]

Add optional board prop viaPlugging to select the via-hole plugging process for a future manufacturing order. A boolean records whether plugging is requested but cannot identify the process.

JLCPCB’s technical guide, under “Are there any size limits for plugged vias?”, groups ink, epoxy, and copper paste as plugging processes. The prop preserves their distinct processes:

Prop value	JLCPCB process
solder_mask_ink	Plugged: soldermask ink plugging with soldermask coverage
epoxy_filled_and_capped	Epoxy-Filled & Capped
copper_paste_filled_and_capped	Copper-Paste-Filled & Capped

Tented and Untented are not values of this prop. defaultViaTenting remains the existing tenting control; ink plugging and filled-and-capped processes are not treated as physically identical.

Values are preserved without aliases or automatic defaults. Omission leaves the process unspecified, and existing boards require no migration. Core will serialize the selection as pcb_board.via_plugging from tscircuit/circuit-json#846 and check for explicit exposed board defaults conflicting with the ink-plugging option. This supersedes the boolean-only proposal in #866.

Generated documentation is updated. Renderer changes, fabrication-rule validation, and ordering integration are outside this Props PR.

Validation: 575 tests pass, including two focused tests for all three processes, stampboard inheritance, omission, tenting preservation, and invalid inputs. Typecheck, build, formatting, and circular dependency checks pass.

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