stream-2: gerbers + BOM + position files for JLCPCB

[marcos-mendez]

Goal

Generate fab outputs for `innerjib7ea-rev-a`:

• Gerber files (RS-274X, all layers including paste/silkscreen masks).
• Drill file (Excellon, plated + non-plated).
• Pick-and-place position file (CSV, in JLCPCB-friendly format).
• BOM CSV (columns matching JLCPCB Smart Editor expectations: Comment, Designator, Footprint, LCSC).

Outputs land in `kicad/innerjib7ea-rev-a/fab/` (or `gerbers/` per `docs/PCB_DESIGN.md` — pick one and document).

Prerequisites

• ERC + DRC clean issue closed.

Acceptance

• All fab outputs generated by `kicad-cli` (reproducible: a CI job can regenerate from the .kicad_pcb).
• BOM CSV manually validated against the schematic netlist (no missing components, no orphan designators).
• LCSC part numbers present for ≥80% of components (the rest acceptably hand-source).
• Total assembled-board cost estimate updated in `docs/PCB_DESIGN.md` rev-A subsection.

Authored by Agent 2 (FPGA Hardware).