feat: PORT-to-INTERCONNECT conversion (+ case-insensitive enum CLI options)

[KelvinChung2000]

Summary

Two related changes, one per commit:

1. fix(cli) — enum-valued CLI options now accept any case.
2. feat(transform) — new port-to-interconnect transform + CLI command.

fix(cli): case-insensitive enum option values

EntryType / ConflictStrategy member values are lower case, so the natural
upper-case SDF spelling crashed with a raw traceback:

$ sdf-toolkit query design.sdf --entry-type IOPATH
ValueError: 'IOPATH' is not a valid EntryType

A CaseInsensitiveStrEnum base resolves values case-insensitively via
_missing_, so --entry-type IOPATH (and --strategy KEEP-FIRST) work.
Genuinely invalid values now surface as a clean typer.BadParameter instead of
a traceback.

feat(transform): PORT → INTERCONNECT

Back-ends such as Microchip Libero express routing delay with (PORT <pin> ...),
which names only the load pin and leaves the driver implicit. Tools that consume
only (INTERCONNECT <src> <dst> ...) (both endpoints named) cannot use those
delays. port_to_interconnect rewrites each PORT into the equivalent
INTERCONNECT by recovering the driver from the gate-level netlist.

sdf-toolkit port-to-interconnect design.sdf netlist.v -o design_interconnect.sdf

Drivers are read from connectivity, never guessed. Yosys parses the netlist
into bit-level nets (reusing the path already used for INTERCONNECT annotation);
the SDF's own IOPATH outputs and the top-level port directions identify drivers;
a cell the SDF does not time is resolved by elimination on its single-driver net
(the one endpoint that is not a known sink). Any sink that does not resolve to
exactly one driver raises DriverResolutionError with the full list — nothing is
dropped or defaulted.

Testing

• tests/test_interconnect.py: hand-built-netlist unit tests (no Yosys) covering
  top-input drivers, IOPATH-identified cell-output drivers, elimination for an
  untimed buffer, PORT removal, top-cell placement, delay carry-over, input
  immutability, and the raising paths (missing/unknown top module, instance not
  in netlist, ambiguous driver). Plus Yosys-gated integration + CLI tests over a
  new tests/data/port_netlist.{v,sdf} fixture (+ golden).
• tests/test_typer_cli.py: upper-case --entry-type works; invalid value is a
  clean error, not a traceback.
• Full suite green (346 passed); new/changed files pass ruff check and
  ruff format.

Notes

• Pre-existing ruff findings in analysis/* and the top-level __init__.py
  are left untouched (out of scope for this PR).