fix: respect constAsEnum in all const-emitting code paths(#588)

[won-N-only]

Fixes #588

Problem

constAsEnum is only respected in one of the three places that emit a single fixed value:

location	path	respects
typescript-json-schema.ts:776	literal type	yes
typescript-json-schema.ts:973	enum member type	no
typescript-json-schema.ts:1035	union collapsing to one value	no

So const still leaks out with the flag on, and the document is not valid OpenAPI 3.0:

interface MyObject {
    reference: true;   // { "enum": [true] }  ok
    member: Enum.X;    // { "const": 0 }      expected { "enum": [0] }
}

What changed

The const-vs-enum decision moves into one setSingleValue helper used by all three sites, so they cannot drift apart again. It returns the definition, which keeps the union site's original ternary.

Two test cases added — an enum member type, and the reproduction from #588. Both fail on branch master.

Impact

None when constAsEnum is off, which is the default. The existing enums-value-in-interface test covers the same enum member shape with the flag off, still expects const, and passes unchanged.

---

• Make your pull request atomic, fixing one issue at a time unless there are many relevant issues that cannot be decoupled.
• Provide a test case & update the documentation in the Readme.md
Readme.md is unchanged: --constAsEnum is already documented there and this makes the implementation match it.