Feature/robust checks

[tbittar]

Description

This PR strengthens correctness guarantees of the library-resolution and simulation pipeline in three areas: the linearity check in constraints and objectives is generalized from dual/reduced_cost detection to a proper non-linearity test; two new semantic rules on port-field expressions are enforced at library-resolve time; and the system-level `sum_connections` robustness check is made more general.

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Behavior changes

1. Constraint / objective linearity check — `_forbid_dual_or_rc` → `_forbid_nonlinear`

The previous guard (`contains_dual_or_reduced_cost`) only caught `dual()` and `reduced_cost()`. It is replaced by `is_linear()`, which correctly rejects all non-linear expressions.

Expression in a binding constraint or objective	Before	After
`dual(c)`	`ValueError` (detected as dual/rc)	`ValueError` (non-linear)
`reduced_cost(x)`	`ValueError` (detected as dual/rc)	`ValueError` (non-linear)
`abs(x)` with `x` a variable	Accepted (not dual/rc)	`ValueError` (non-linear)
`max(x, y)` with variables	Accepted (not dual/rc)	`ValueError` (non-linear)
`abs(a)` / `max(a, b)` with parameters only	Accepted	Accepted (constant, degree 0)

The same generalisation is applied in `optimization.py`: port-field definitions that are non-linear (not just dual/rc) are now correctly skipped during the LP build phase and deferred to the extra-output post-solve evaluator.

2. New rule: bare `port.field` is not allowed outside `sum_connections`

A bare `PortFieldNode` (e.g. `balance_port.flow`) in any binding constraint, plain constraint, extra-output, or objective contribution now raises a `ValueError` at `resolve_library()` time. `sum_connections(...)` is not recursed into, so the `PortFieldNode` inside remains valid.

Expression	Context	Before	After
`balance_port.flow >= 0`	binding constraint	Accepted (degree 1, linear)	`ValueError`
`balance_port.price >= 0`	binding constraint	Accepted	`ValueError`
`balance_port.flow = withdrawal`	plain constraint	Accepted	`ValueError`
`sum_connections(balance_port.flow) >= 0`	any	Accepted	Accepted (no change)

3. New rule: `sum_connections` cannot refer to a port field defined in the current model

If a model's `port-field-definitions` already provides a definition for `port.field`, any expression that contains `sum_connections(port.field)` — in a binding constraint, plain constraint, extra-output, or objective — now raises a `ValueError` at `resolve_library()` time.

Expression	Condition	Before	After
`sum_connections(p.f)` in a binding constraint	`p.f` defined in this model	Silently accepted	`ValueError`
`sum_connections(p.f)` in an extra-output	`p.f` defined in this model	Silently accepted	`ValueError`
`sum_connections(p.f)` in a constraint	`p.f` defined in this model	Silently accepted	`ValueError`
`sum_connections(p.f)` anywhere	`p.f` not defined in this model	Accepted	Accepted (no change)

4. System-level `sum_connections` robustness — `_check_no_dual_rc_sum_connections` → `_check_linear_sum_connections`

The system-level guard that prevents aggregating a non-linear port-field definition via `sum_connections` previously only detected `dual()`/`reduced_cost()`. It now uses `is_linear()`, catching all non-linear definitions.

Master port-field definition	Before	After
Contains `dual()` or `reduced_cost()`	`ValueError` at connection time	`ValueError` at connection time
Contains `abs(x)`, `max(x,y)`, etc. on variables	Silently accepted (wrong)	`ValueError` at connection time
Linear	Accepted	Accepted (no change)

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Other changes

• `predicates.py` removed; `uses_sum_connections_on.py` (renamed from `predicates.py`) is now the single-purpose module for that visitor.
• `port_resolver.py` removed (superseded).
• `model_port_definition_ok.yml` and `model_port_definition_ko.yml` fixtures updated: bare port field reference replaced with `sum_connections(...)`, consistent with Rule 2.
• New tests in `test_lib_parsing.py`: acceptance/rejection matrix for all expression contexts (variable bounds, objectives, constraints, extra-outputs), plus 6 dedicated tests for the two new port-field rules.
• New tests in `test_components_parsing.py`: `sum_connections` linearity checks at connection time.

Checklist

• Unit tests pass (`pytest`)
• Type checking passes (`mypy`)
• Formatting passes (`black`, `isort`)
• `pyproject.toml` version bumped if applicable
• `AGENTS.md` reviewed for impact and updated if needed