Avoid file read buffer copy

[SaladDay]

Bounded file reads accumulated bytes as a string and then copied the full result into a byte slice. Use a standard-library byte buffer and return its owned bytes, preserving empty nonnil results, partial error results, size limits and handle/reference ownership. No RPC, cache, protocol or dependency is added.

Six alternating paired benchmarks through the actual File client/server show lower allocation for nonempty files: 4 KiB reads save about 4 KiB and one allocation; 1 MiB reads reduce allocated bytes from 9.29 MB to 5.59 MB and local median latency from 2.48 ms to 1.50 ms. Larger gains include buffer growth differences. Empty reads add 16 B/op; RPC counts are unchanged. These are local function/protocol results, not live Session speedup claims.

Validation: package tests including manifest coverage, six read-result semantics cases, race, vet, Runtime build and hygiene passed. Existing privileged-view tests were gated off. Coordinator reviewed the complete diff; same-source live qualification follows integration.

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