Separate a nested implementation with a dot, not a dollar

[pjfanning]

Following jackson-module-scala#839.

An implementation nested in a class other than the root was written {"@type":"Boxed$Same"}. It is now {"@type":"Boxed.Same"} — the way jackson-databind separates a name from what encloses it. This changes the JSON, so it is a wire-format change, not a refactor.

public sealed interface Dup extends SealedPolymorphismSupport permits Boxed.Same, Nested.Same {}

class Boxed  { record Same(int v)    implements Dup {} }   // {"@type":"Boxed.Same","v":1}
class Nested { record Same(String v) implements Dup {} }   // {"@type":"Nested.Same","v":"x"}

Only real nesting becomes a dot

A $ is a legal character in a Java identifier, so a class can carry one in a name of its own making. The enclosing-class chain is what tells the two apart: a class with a $ in its own name has no enclosing class at that position, so that $ is left alone.

This is the counterpart of the Scala module's concern with :: compiling to $colon$colon. There is a fixture named Odd$Name asserting such a name survives intact:

{"expr":{"@type":"Odd$Name","head":1,"tail":2}}
{"expr":{"@type":"Grouped.Inner","v":5}}

Simpler here than in Scala

The Scala module has to convert the dot back to a $ before looking a name up, because it resolves by rebuilding candidate class names and calling Class.forName. Nothing of the sort is needed here — names are resolved against the hierarchy's own table, built from the PermittedSubclasses attribute, so the derived name is simply the map key. A dot in a @type still cannot address a package, and is still never handed to Class.forName.

So this PR changes typeNameFor and nothing else in the main sources.

Tests

93 total. Three existing name-shape assertions updated (SealedPolymorphismTest, SealedTypesTest, MixInTest), plus three new ones: the dot for an enclosing class, a $ that is part of the class name surviving, and typeNameFor covering both directly.

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https://claude.ai/code/session_012hxYY9KPDAMXADWzjBHNV4