Depend on halfphi instead of keeping a second copy of it #3

crates/halfphi/ here and https://github.com/tinymachines/halfphi hold the
same five files, and nothing keeps them in sync.

They drifted within minutes of the split, on whitespace: the standalone repo runs
cargo fmt --check in CI, this repo has never been fmt'd, and default rustfmt
reflows the one-line struct literals used throughout this codebase. That is
patched over with a shared rustfmt.toml, and tests/chips.rs searches two
candidate paths for the die submodule so one file can be correct in both
layouts. Both are stopgaps.

The five files are byte-identical today. Verify with:

for f in src/lib.rs src/netlist.rs src/engine.rs src/source.rs tests/chips.rs; do
  diff -q "crates/halfphi/$f" "../halfphi/$f"
done

The fix

Replace halfphi = { path = "../halfphi" } with a git dependency (pinned to a
tag or rev) or a crates.io version once published, and delete crates/halfphi/
from this workspace.

Why now

It is cheapest before the APIs diverge. Once someone fixes a parser bug in one
copy and not the other, the two stop being the same library and every claim made
about one stops applying to the other — which is exactly the class of quiet
difference this project exists to remove.

Acceptance

• One copy of the source, in one repository.
• cargo test --workspace still passes, including the golden differential.
• All six derived artifacts (layout.bin, blueprint/blocks/schematic/decode/ timing.json) remain byte-identical.

See also tinymachines/halfphi#4.