Lossless BNG-XML \xe2\x86\x92 BNGL round-trip for tfun, rule modifiers, selectors

[wshlavacek]

Summary

Three related fidelity fixes when reading BNG2.pl-emitted BNG-XML and re-emitting BNGL. Each fixes a specific shape that the parser currently drops, mangles, or warns about and ignores.

1. `tfun` placeholder reconstruction

BNG2.pl serializes `tfun(...)` calls — both the inline-array form `tfun([xs],[ys],ctr)` and the file-based `TFUN(ctr,"file")` form — by replacing the function body with a `TFUN_VAL` / `TFUN__VAL` placeholder and stashing the real arguments on attributes of the `` element (`@ctrName`, `@xData`, `@yData`, `@method`, `@file`).

`FunctionBlockXML.parse_xml` was reading the placeholder verbatim into the regenerated BNGL, which BNG2.pl can't re-parse. New `_resolve_expression` helper reconstructs the call from the attributes.

2. `RuleMod` supports multiple modifiers per rule

A single rule can legitimately carry, say, `DeleteMolecules` together with `include_reactants(...)`. `RuleMod.modifiers: list[str]` stores the BNGL serialization of each modifier in insertion order. `str` prefers the list when non-empty, falling back to the single `self.type` so existing callers that read `rule_mod.type` keep working.

3. `include` / `exclude` reactants / products as rule modifiers

These are now emitted as rule modifiers instead of being dropped with a stderr warning. BNG2.pl serializes them as `ListOfInclude{Reactants,Products}` / `ListOfExclude{Reactants,Products}` children on the rule, each carrying a pattern-index suffix (`_RP` / `_PP`) on the selector id and one or more `` entries. New helpers `_build_selector_modifier` and `_format_selector_pattern` mirror those back as e.g. `include_reactants(1, MyMol)`.

The `get_rule_mod` body is also tightened: it now tolerates rules that have no `ListOfOperations` at all (previously `KeyError`) and returns `None` only when there's genuinely no modifier to report, instead of dropping a real one when `ListOfOperations` happens to be missing.

Note on conflict with #76

This PR and #76 both modify the import block of `xmlparsers.py` to add `import re`, and both modify `FunctionBlockXML.parse_xml` to wrap the `Expression` read. The two rewrites compose — `_decode_xml_boolean_ops(_resolve_expression(...))` is the combined call — but I've kept them as separate PRs since the audit's table 0.4 split them. Whichever lands first, the other rebases trivially.

Test plan

• Existing CI passes
• A model using `tfun(...)` round-trips through XML and BNG2.pl re-parses the regenerated BNGL
• A rule using `include_reactants(...)` round-trips through XML
• A rule combining `DeleteMolecules` with `include_reactants(...)` emits both modifiers