chore(cad_core): add units tolerances tests

[Obayne]

Title: chore(cad_core): add units tolerances tests

Summary

• Adds focused tests for cad_core/units.py covering EPS, almost_equal, clamp, sgn, and round_tol.
• Introduces cad_core/units.py helpers with improved round_tol using Decimal to avoid FP drift.

Changes

• New: tests/cad_core/test_units_tolerances.py
• New: cad_core/units.py
• Task: tasks/chore-cad-core-units-tolerances-tests.md

Rationale

• Establish consistent tolerance behavior used across core geometry.
• Provide small, pure tests to keep regressions visible and fast.

Test Plan (agents pulling this)

• From repo root:
  • python -m pip install -e . (or set PYTHONPATH to repo root)
  • ruff check cad_core tests/cad_core/test_units_tolerances.py
  • black --check cad_core tests/cad_core/test_units_tolerances.py
  • pytest -q tests/cad_core/test_units_tolerances.py

Notes

• No UI impact; pure functions only.
• Keep EPS and helpers as the single source of tolerance logic across CAD core.

Refs

• Issue: N/A (please update if applicable)