feat: generics multiple-bound conjunctions (ILO-386)

[danieljohnmorris]

Summary

• Extends bounded generics (ILO-61, feat: bounded generics for sound polymorphism (ILO-61) #620) to support conjunctions of bounds with + syntax: <a:numeric+comparable> means a must satisfy ALL listed bounds (intersection, not union).
• Builds on ILO-385 (generics: return-type substitution at call sites (ILO-385) #628) generic return-type substitution.

Changes

src/ast/mod.rs: type_params field changed from Vec<(String, Bound)> to Vec<(String, Vec<Bound>)> — each type variable carries a conjunction of bounds.

src/parser/mod.rs: After consuming the first bound name, loops consuming + + another bound ident to build the conjunction. No tokenizer changes needed — Token::Plus already existed.

src/verify.rs: type_bounds map now holds Vec<Bound>; call-site bound checker iterates all bounds in the conjunction and emits ILO-T044 for each unsatisfied one.

tests/regression_generics_multibound.rs: 10 regression tests covering:

• Acceptance: n satisfies numeric+comparable
• Rejection: t fails numeric in numeric+comparable
• Rejection: b fails both bounds in numeric+text
• Three-bound impossible conjunctions (numeric+comparable+text)
• Order independence (comparable+numeric behaves same as numeric+comparable)
• Regressions: single-bound and unbounded type vars still work

Test plan

• cargo test --test regression_generics_multibound — 10/10 pass
• cargo test --lib — 3342/3342 pass
• Full integration test suite

Closes ILO-386

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