fix: clarify copy fast-path contracts

[hayabusa-cloud]

Summary

This PR aligns the iox copy contract docs, fallback benchmarks, and validation timeouts with the implementation. It keeps invalid Writer counts as a caller-owned contract violation instead of adding repeated hot-loop validation branches.

Changes

• Clarifies Copy and CopyPolicy fast-path boundaries: WriterTo and ReaderFrom implementations own source advancement, partial-write recovery, semantic error preservation, and progress accounting.
• Replaces stack-buffer wording with implementation-accurate internal fixed-size buffer wording across GoDocs and READMEs.
• Updates slow-path copy benchmarks so they bypass WriterTo and actually exercise the generic read/write loop.
• Adds 120 second timeouts to CI and Makefile validation commands.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

This PR updates iox’s Copy-family documentation and benchmarks to more precisely describe (and measure) behavior around the io.WriterTo / io.ReaderFrom fast paths, and adds explicit go test timeouts in local/CI automation.

Changes:

• Clarifies fast-path contract boundaries in GoDocs and READMEs (fast path owns advancement, partial-write recovery, and semantic error preservation).
• Fixes “slow-path” benchmarks to actually exercise the generic read/write loop (by using a Reader-only source).
• Adds -timeout 120s to go test invocations in CI and Makefile targets.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
io.go	Expands Copy/CopyPolicy docs to spell out fast-path ownership; updates buffer wording to “internal fixed-size buffer”.
doc.go	Refines package-level semantics docs to make fast-path responsibilities explicit.
bench_test.go	Adds a Reader-only benchmark source and switches slow-path benchmarks to avoid WriterTo fast path.
README.md	Aligns English README fast-path and buffer wording with implementation/contract.
README.zh-CN.md	Mirrors README fast-path/buffer clarifications in zh-CN translation.
README.ja.md	Mirrors README fast-path/buffer clarifications in Japanese translation.
README.fr.md	Mirrors README fast-path/buffer clarifications in French translation.
README.es.md	Mirrors README fast-path/buffer clarifications in Spanish translation.
Makefile	Adds -timeout 120s to go test targets (tests and benchmarks).
.github/workflows/ci.yml	Adds -timeout 120s to CI test command and normalizes formatting.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.