fix(mem): implement io.ReaderFrom on MemMapFs files

[leno23]

Summary

• Implements io.ReaderFrom on mem.File so MemMapFs files behave like *os.File for io.Copy fast paths.
• ReadFrom writes starting at the current file offset and advances it, matching os.File semantics.
• Adds unit tests in mem and integration tests for MemMapFs.

Fixes #498

Test plan

• go test ./...

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[spf13]

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