Use pcb_board DRC values and remove JLC specs dependency import

[imrishabh18]

Motivation

• Remove the hard dependency on @tscircuit/jlcpcb-manufacturing-specs and instead use DRC values embedded in the pcb_board element of the circuit JSON.
• Allow board-level manufacturing DRC properties to override library defaults so checks reflect per-board constraints.

Description

• Removed @tscircuit/jlcpcb-manufacturing-specs from devDependencies and replaced dependency-backed defaults with local numeric fallbacks in lib/drc-defaults.ts.
• Added getBoardDrcValue(circuitJson, key, fallback) to read manufacturing DRC properties from the pcb_board element and return a typed numeric fallback when unset.
• Updated clearance/thickness fallbacks so checks read from board DRC properties when minClearance or trace widths are not explicitly provided: pad-pad uses min_pad_to_pad_clearance, pad-trace and via-trace use min_trace_to_pad_clearance, via spacing uses min_via_to_via_clearance, and trace thickness falls back to min_trace_width.
• Updated via off-board checks to use min_board_edge_clearance (with fallback) for the board-edge margin.

Testing

• npm run build completed successfully.
• Ran npx biome format --write on all touched files (succeeded) to apply formatter fixes.
• npm run format:check initially reported repository-level warnings due to large test fixtures, and formatter suggestions were applied to the modified files; formatting of touched files is now applied.

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Codex Task