feat: preflight RootFS storage capacity

[jacklv-coder]

Summary

• calculate same-volume RootFS installation headroom before creating staging: compressed snapshot + temporary decompressed tar + materialized payload + 16 MiB reserve
• return a typed insufficient-storage error with required/available bytes while allowing valid existing installations to be reused without reinstall capacity
• use the wider of manifest and custom-extractor expanded bounds and reject integer overflow
• verify low-space upgrades preserve the current installation and inject ENOSPC after both destructive promotion checkpoints to prove rollback
• update the bilingual architecture, implementation, RootFS, testing, roadmap, and changelog facts

Validation

• swift test — 128 tests, 0 failures
• strict-concurrency build with warnings as errors
• exact v0.3.3 RootFS first-materialization test
• documentation mirror/link/anchor checks
• shell syntax and git diff --check
• arm64 Simulator final link
• unsigned arm64 device final link
• Demo build
• iOS 18.2 public ABI.4 RootFS smoke — 14/14 checks
• Codex CR of the complete diff — custom-extractor underbudget finding fixed; no remaining P1/P2

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