Migrate synthesis-method registry to shared synthesis-methods package

[micah-olivas]

What

Migrates uSort-M off its own copy of the synthesis-method registry and onto the standalone synthesis-methods package (private repo micah-olivas/synthesis-methods, also consumed by library_designer), so method definitions live in one place. Cost computation stays in uSort-M. No cost math or pricing numbers change.

Changes

• src/usortm/costs/method_loader.py — now a thin shell that re-exports the registry (METHODS_DIR, SynthesisMethod, load_method, load_all_methods, find_methods) from synthesis_methods, and keeps compute_cost + the four _compute_* helpers unchanged (uSort-M–specific).
• pyproject.toml — adds the synthesis-methods dependency; drops the now-unused tomli backport (TOML parsing lives in the package now).
• Deletes src/usortm/costs/methods/ — the 8 method TOMLs + sources/ (the package owns them; 559 deletions).

Back-compat

Every existing from usortm.costs.method_loader import … keeps working via the re-export — consumers (cost_functions.py, cli/plan.py, cli/estimate.py, docs/generate_cost_data.py, tests/test_method_loader.py) are untouched.

Verification

• Import check → load_all_methods() returns 8 methods.
• Cost regression: compute_cost across every method × a (count × length) grid — output byte-identical pre/post (sha256 match).
• pytest tests/test_method_loader.py → 11 passed; full suite → 140 passed.
• estimate --json (library 500 × 300 bp, seeded sim) → byte-identical JSON pre/post.

Dev setup

The package is private/unpublished, so install it editable alongside uSort-M:

pip install -e ../synthesis-methods   # first, so pip sees the requirement satisfied
pip install -e .

Switch the pyproject entry to a git/index source once synthesis-methods is public. Going forward, edit method TOMLs only in the synthesis-methods repo.