fix: add explicit zlib constant fallbacks for esbuild-stripped values

[DawidMyslak]

Summary

• Packages like axios crash inside the V8 isolate with TypeError: Cannot read properties of undefined (reading 'Z_SYNC_FLUSH') because esbuild tree-shakes zlib constants out of the browserify-zlib bundle
• Add explicit fallbacks for all standard zlib.h constants (flush, return codes, compression levels, strategies) matching the existing pattern for mode constants
• Add comprehensive test asserting all 25 constants are present with correct values

Test plan

• Existing zlib polyfill tests continue to pass
• New zlib.constants has flush, return-code, level, and strategy constants test passes in CI
• axios-pass project-matrix fixture passes

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