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LDC-E1337 — Instance breaks the expansion contract

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# LDC-E1337 — Instance breaks the expansion contract

## What it means

An instance fills every slot its template declares: each binder is
bound to a binder parameter, each argument resolves to a value of the
declared shape, and no binder is bound twice. A missing argument, a
binder pointing at a non-binder, or a value that resolves nowhere —
any of these breaks the contract, and the compiler stops at the
offending line.

The repair is to complete the instance: add the missing line, point
the binder at a real binder, or spell a value the package declares. A
complete instance expands exactly as written.

## Example

```law
language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity Person;
entity Choice;
entity SpecialChoice : Choice;
entity Other;
const Selected: SpecialChoice = entity_ref("urn:choice:selected");
const Wrong: Other = entity_ref("urn:choice:wrong");
relation eligible(p: Person, choice: Choice) kind empirical;
relation applicable(p: Person, choice: Choice) kind institutional;
expansion fixed {
    params { subject: binder; choice: value; ground: relation(subject, choice); result: relation(subject, choice); }
    emit rule self/result defeasible {
        for subject;
        when ground(subject, choice);
        then result(subject, choice);
        scope from self; effective from self; labels from self; source from self;
    }
}
expand fixed Result {
    label en official "Fixed option";
    bind subject = person: Person;
    choice = entity_ref("urn:choice:raw");
    ground = eligible;
    result = applicable;
    effective [@2026-01-01, @2027-01-01);
}
```

## Fix

```law
language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity Person;
entity Choice;
entity SpecialChoice : Choice;
entity Other;
const Selected: SpecialChoice = entity_ref("urn:choice:selected");
const Wrong: Other = entity_ref("urn:choice:wrong");
relation eligible(p: Person, choice: Choice) kind empirical;
relation applicable(p: Person, choice: Choice) kind institutional;
expansion fixed {
    params { subject: binder; choice: value; ground: relation(subject, choice); result: relation(subject, choice); }
    emit rule self/result defeasible {
        for subject;
        when ground(subject, choice);
        then result(subject, choice);
        scope from self; effective from self; labels from self; source from self;
    }
}
expand fixed Result {
    label en official "Fixed option";
    bind subject = person: Person;
    choice = Selected;
    ground = eligible;
    result = applicable;
    effective [@2026-01-01, @2027-01-01);
}
```

## Engine message

The engine reports this in its own wording:

```text
example.law:25:5: error LDC-E1337: expand Result: "choice": value is unresolved or incompatible with value<T>
```

## Related

- [LDC-E1336](/diagnostics/LDC-E1336/) — an instance must resolve to exactly one definition.
- [LDC-E1339](/diagnostics/LDC-E1339/) — an instance must carry a label.