Add SDF timing path tracer tool for debugging timing discrepancies

Python script that parses SDF files and traces timing paths through
INTERCONNECT and IOPATH entries. Subcommands: clock-tree (trace clock
buffer path to DFF), trace-back/trace-fwd (generic path tracing),
cell-info (show all SDF data for an instance), output-path (DFF Q to
output port).

Key finding from using this tool: the 242ps systematic offset between
CVC and Jacquard on flash_clk is caused by PnR-inserted output buffers
(e.g. output20, sky130_fd_sc_hd__clkdlybuf4s25_1) that exist in SDF
but not in the synthesis netlist. CVC applies these delays; Jacquard
does not see them.

Co-developed-by: Claude Code v2.1.50 (claude-opus-4-6)

Author: robtaylor