ENH: Convert Hunspell Dictionary affix directive parsing else-if chain to switch

### Description

## Issue Description

In `org.apache.lucene.analysis.hunspell.Dictionary#readAffixFile`, affix directives (such as `PFX`, `SFX`, `CIRCUMFIX`, `KEEPCASE`, `NEEDAFFIX`, etc.) were being checked sequentially using a large `if-else` chain spanning over 120 lines.

The code previously included a TODO comment asking:
```java
// TODO: convert to a switch?
```

Using a modern Java `switch` statement on `firstWord` simplifies this construct, improves code readability, and allows the Java compiler to produce a more efficient bytecode lookup scheme rather than sequential `String.equals()` comparisons.

## Proposed Changes

1. **Refactor `readAffixFile` Directive Parser**:
- Replace the `if ("AF".equals(firstWord)) ... else if ...` chain in `Dictionary.java` with a `switch (firstWord)` statement.
- Combine multi-label cases sharing identical logic (e.g. `"NEEDAFFIX", "PSEUDOROOT"` and `"ICONV", "OCONV"`).
- Removed the stale `// TODO: convert to a switch?` comment.

2. **Refactor Related Directive Helpers**:
- `getDecoderAndFlagParsingStrategy`: Converted `"SET"` / `"FLAG"` header check loop from `if-else` to `switch (firstWord)`.
- `getFlagParsingStrategy`: Refactored `if ("num".equals(flagType))` chain to a concise `switch` expression returning strategy instances.

## Benefits

- **Readability & Maintainability**: Clearer structure and alignment across all Hunspell directive handlers.
- **Performance**: Better bytecode optimization with `tableswitch`/`lookupswitch` over linear string comparisons.
- **Code Cleanliness**: Resolves existing `TODO` comment.

## Verification

Ran all Hunspell analysis tests:
```bash
./gradlew :lucene:analysis:common:test --tests "org.apache.lucene.analysis.hunspell.*"
```
**Result**: Build succeeded, all 158 tests passed.

Start in org.apache.lucene.analysis.hunspell.Dictionary#readAffixFile and inspect the related getDecoderAndFlagParsingStrategy and getFlagParsingStrategy methods. Replace the specified conditional chains with switch forms while preserving directive handling, then run ./gradlew :lucene:analysis:common:test --tests "org.apache.lucene.analysis.hunspell.*". Done means the refactor is complete, the stale TODO is removed, and all 158 Hunspell tests pass.

Written by the indexing model from the issue text.