test: cover RootFS write-stage ENOSPC

[jacklv-coder]

Summary

• add deterministic write fault seams for RootFS snapshot, gzip output, tar payload, installation record, promotion journal, and current record
• exercise real partial gzip output cleanup and preserve the prior RootFS/current.json across every injected ENOSPC stage
• normalize POSIX-backed write descriptions and update bilingual architecture, implementation, testing, security, roadmap, and changelog docs

Validation

• swift test — 130 tests, 1 expected missing-environment RootFS skip, 0 failures
• exact write-stage ENOSPC tests — pass
• real pinned v0.3.3 RootFS first materialization — pass
• swift build --target PocketRootResources -Xswiftc -strict-concurrency=complete -Xswiftc -warnings-as-errors — pass
• Scripts/check-docs.sh, bash -n Scripts/*.sh, and git diff --check — pass
• Scripts/build.sh and Scripts/build-runtime-spike.sh — pass
• Codex CR of the uncommitted exact diff — no correctness findings / no P1 or P2

Known unrelated validation gap

A full-package strict-concurrency warnings-as-errors test reaches an existing Swift 6.2 Task capture diagnostic in PocketRootOpenAIResponsesClientTests.swift:391; the changed PocketRootResources target passes the same strict flags, and the ordinary full suite is green.