feat(bngsim): evaluate simulate/parameter_scan action args as expressions

[wshlavacek]

What

Numeric arguments of simulate(...) and parameter_scan(...)/bifurcate(...)
actions can now be arithmetic expressions (e.g. t_end=>2*5), not just bare
numbers.

Why

BNGL action arguments are parsed as raw strings (_parse_action_value keeps
scalars as text), so an arithmetic value like t_end=>2*5 reached the handlers
as the string "2*5" and was handed to bare float()/int(), which raised
ValueError. Routing every numeric argument through _eval_numeric — the same
no-builtins safe evaluator setParameter/setConcentration already use — makes
an expression resolve exactly the way its literal equivalent does.

Plain numbers are unchanged (_eval_numeric tries float() first, so results
are byte-identical). This is purely additive — no existing behavior is removed.

Converted in both handlers:

• _prepare_simulate_run: continue, t_start, t_end, n_steps,
  print_functions, atol, rtol, seed, steady_state
• _resolve_scan_settings: t_start, t_end, steady_state,
  print_functions, seed, reset_conc

Test plan

New tests/test_bngsim_action_expressions.py:

• simulate: t_end=>2*5, n_steps=>4*5 reproduces the literal 10/20
  run exactly (real bngsim engine, two fresh models compared point-for-point).
• parameter_scan: expression-valued t_start/t_end/print_functions/
  reset_conc/seed resolve to the expected numbers.
• Verified the new tests fail on the pre-change code
  (ValueError: could not convert string to float: '1+1').

Full bngsim suite green with BNG2.pl (BioNetGen 2.9.3): 312 passed on this
branch in isolation.

🤖 Generated with Claude Code