refactor: migrate ECC workspace paths to pathlib

[Emin017]

Summary

• Move ECC runtime filesystem paths toward pathlib.Path / Path | None internally.
• Preserve string-based contracts at CLI, JSON, subprocess, and native binding boundaries.
• Fix boundary regressions found during the migration, including None path serialization and normalized workspace create responses.

Scope

Select the areas touched by this PR:

• CLI - command behavior, Typer command surface, output formats, or workspace commands.
• Flow/runtime - workspace lifecycle, EngineFlow, step execution, logs, metrics, or artifacts.
• EDA integration - Yosys, ECC-Tools, DreamPlace, KLayout, PDKs, or native/runtime wrappers.
• Build/package - Bazel, Nix, PyInstaller, wheels, uv.lock, or release artifacts.
• CI/release - GitHub Actions, version checks, changelog, or release automation.
• Tests/docs only

What Changed

• Store and pass workspace, PDK, parameter, home, checklist, JSON helper, and step builder filesystem values as Path objects internally.
• Keep public APIs such as create_workspace(), load_workspace(), and get_pdk() string-compatible, normalizing paths at the boundary.
• Convert paths back to strings before JSON writes, CLI/service responses, subprocess calls, and ECC native binding calls.
• Preserve existing workspace JSON schemas and artifact filenames.
• Add behavior-focused tests for Path storage, public string compatibility, JSON serialization, native/subprocess stringification, missing Sizer/DreamPlace inputs, and normalized
  workspace create responses.
• Remove signature-only assertion tests so coverage checks runtime behavior instead of annotation text.

Validation

List the commands you ran. Mark checks that are not applicable as N/A.

• uv run pytest test/
• Focused pytest:
• uv run ruff check chipcompiler test
• uv run ruff format --check chipcompiler test
• bazel build //:build_ecc_cli_bundle
• PyInstaller smoke: ecc --help, ecc --version, ecc version --json
• Nix smoke: nix run .#cli -- --help
• Manual flow smoke:
• Other:

Skipped checks and reason:

Runtime And Packaging Impact

• No runtime or packaging impact
• CLI output or machine-readable contract changed
• Workspace layout, flow state, or artifact paths changed
• Native toolchain or wrapper behavior changed
• ecc-tools or ecc-dreamplace dependency changed
• PyInstaller, Nix, Bazel, or release artifact changed

Notes:

Checklist

• I kept the change scoped to ECC.
• I updated docs or user-facing CLI text where behavior changed.
• I included lockfile or version metadata updates when dependencies changed.
• I documented any submodule updates and why they are needed.
• I did not include local caches, virtual environments, or generated build outputs.
• I explained skipped validation and remaining risk.