Fix static using leakage and qualify nested enum static usings (#148)

[paulirwin]

Fixes #148.

The reported bug

StaticUsingEnumNames lived on JavaConversionOptions, which callers reuse across every file in a conversion run, and was never cleared. Each successive file emitted the static usings of every enum seen before it.

The leak was also cross-namespace: UsingsHelper prefixes each name with the current file's namespace, so an enum from com.foo surfaced in a com.bar file as using static Com.Bar.Color; — a reference to a type that does not exist there.

Rather than a try/finally around ConvertText, this moves the accumulator to ConversionContext, which is already constructed once per ConvertText call and already holds the other per-file state (PendingAnonymousTypes, UsedAnonymousTypeNames). That scopes the state correctly by construction instead of relying on cleanup, and is safe when one options instance is shared across concurrent conversions.

A second, more serious bug found while fixing it

Nested enums emitted a static using that named only the enum, omitting its declaring type. This needed no multi-file setup — a single file was enough:

package com.example;
public class Holder { public enum Inner { A, B } }

using static Com.Example.Inner;   // the C# type is Com.Example.Holder.Inner

That using does not resolve, so any file containing a nested enum generated C# that failed to compile. The name is now built by walking the AST's enclosing type declarations, which also handles arbitrary nesting depth (Outer.Middle.Deep) and enums declared in interfaces.

Related cleanup

• ConversionState moved off options. It was per-run state written on a caller-owned config object — the same defect shape as StaticUsingEnumNames. State changes now go through the context, which records the value and raises the existing StateChanged event, so event ordering is unchanged. An audit of the rest of context.Options.* found only legitimate config reads.
• GetUsings' redundant JavaConversionOptions parameter removed. Its only caller passed the same instance already reachable via context.Options, so the two could not disagree; reading it from the context also lets the internal null-guards go away.

Breaking changes

Public API removals, intended for the upcoming major version bump:

Removed	Notes
JavaConversionOptions.StaticUsingEnumNames	Only ever written internally; not usable as an input.
JavaConversionOptions.ConversionState	Written but never read anywhere in the repo; the GUI tracks state via the StateChanged event.
UsingsHelper.GetUsings(…, JavaConversionOptions?, …)	Parameter dropped; read from context.Options.

Verification

NestedEnumStaticUsing.java is added to the existing integration harness, which Roslyn-compiles and executes the generated C# — it fails to emit before this change.

	Before	After
Cross-file leak tests (2)	fail	pass
Nested-enum tests (4)	fail	pass
NestedEnumStaticUsing integration	fails to compile	pass
Full suite	—	256 pass, 0 warnings

GUI and CLI both build clean against the removed properties.

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