Inline external-sorting module to fix ESM usage

[cmdcolin]

This crashed on windows, which used the non-GNU sort implementation of external-sorting but this package was badly configured for ESM usage

example error from windows jbrowse desktop

    [Browser SEVERE] file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/main.b79dc5be.js 197830:16 TypeError: external_sorting__WEBPACK_IMPORTED_MODULE_0__ is not a function
    at sortLinesExternal (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/3939.cdf1e41e.chunk.js:74597:9)
    at makeIxWithJsSort (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/3939.cdf1e41e.chunk.js:19428:93)
    at makeIxStream (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/3939.cdf1e41e.chunk.js:19437:11)
    at ixIxxStream (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/3939.cdf1e41e.chunk.js:76090:74)
    at runIxIxx (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/5598.5f75b254.chunk.js:53709:42)
    at indexDriver (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/5598.5f75b254.chunk.js:53608:9)
    at perTrackIndex (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/5598.5f75b254.chunk.js:53542:11)
    at indexTracks (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/5598.5f75b254.chunk.js:53498:35)
    at TextIndexRpcMethod.execute (file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/5598.5f75b254.chunk.js:53729:11)
    at file:///D:/a/jbrowse-components/jbrowse-components/products/jbrowse-desktop/dist/win-unpacked/resources/app.asar/build/static/js/5598.5f75b254.chunk.js:30675:12

This PR inlines external sorting and adds extra testing to ensure it works

[cmdcolin]

AI summary:

Bug Fixes

• Buffer concatenation (only appending actual bytes read)
• Division by zero guard in optimizePrefixSize
• Exit codes in bin.ts
• Error propagation with Promise.race in makeIx.ts

Resource Management

• FileParser now keeps file handle open (was reopening on every call)
• Temp directory cleanup in sortLinesExternal
• Try-finally for file handles

Stream Improvements

• Replaced .pipe() chains with pipeline() for proper error handling
• Simplified single-file merge case (15 lines → 3 lines)

Code Simplifications

• Removed unused sortLinesInMemory
• Simplified compare and swap functions
• Moved PassThrough to static import
• String concatenation → join() patterns