fix: copy default read converter map to isolate per-reader custom converters

[Aias00]

Summary

Fixes #971.

The workbook-level read holder aliased DefaultConverterLoader's shared static allConverter map instead of copying it, so custom converters registered via registerConverter(...) were put() into the global map and leaked into every later, unrelated read on the same JVM. This mirrors the write side (AbstractWriteHolder:271), which already copies.

Root cause

AbstractReadHolder:

if (parentAbstractReadHolder == null) {
    setConverterMap(DefaultConverterLoader.loadDefaultReadConverter()); // shared static, no copy
} else {
    setConverterMap(new HashMap<>(parentAbstractReadHolder.getConverterMap()));
}

loadDefaultReadConverter() returns the static allConverter map by reference, so the getConverterMap().put(...) loop right below mutates the shared map and the registration survives the reader.

Fix

setConverterMap(new HashMap<>(DefaultConverterLoader.loadDefaultReadConverter()));

Verification

Added ReadConverterIsolationTest which:

1. reads a file with a registered marker converter → values carry the marker (expected);
2. reads the same file with a fresh reader and no converter → asserts values carry no marker.

Before the fix, step 2 failed (hello [MARKER] instead of hello). After the fix it passes. Full fesod-sheet suite: Tests run: 668, Failures: 0, Errors: 0.

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