Complete sig-edge registration with technical approach diagrams

[kevinzs2048]

sig-edge was already listed as a planned SIG in the README tables and had a charter under sigs/_planned/, but was missing the fep/sig-edge/ directory that every other registered SIG has. This completes that registration and fills in the edge technical approach.

• Add fep/sig-edge/ (with images/) so edge FEPs have a home, matching the sig-os / sig-riscv registration pattern.
• Expand sigs/_planned/sig-edge.md with a "Technical Approach (Draft)" section: platform onboarding matrix, three-layer onboarding path, development-to-deployment flow, and the merged ExecuTorch x TLE / llama.cpp x Triton deployment paths, with five diagrams.
• Refine the sig-edge focus to "edge hardware and edge-side SDK" across fep/README.md, fep/README_CN.md, and sigs/_planned/README.md, and update the charter title accordingly.

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