Add intrinsically isolated mutation batch previews

[JSv4]

Summary

• run atomic or explicit best-effort previews on a complete isolated session clone, never on the live package or its history
• return predicted versions, per-step receipts, semantic revision/comment/annotation deltas, deterministic package-content hashes, warnings for generated metadata, and optional scoped/full HTML
• carry the same preview contract through Ops/JSON, WASM/npm, Python/stdio, and MCP while preserving legacy batch aliases
• preserve mutable settings, revision/transaction generators, and the original diff baseline without sharing caches or undo/redo state
• validate isolation with normal and anchor-persisting OPC payload maps, intentionally ignoring ZIP entry timestamps/compression metadata in line with GetPackageContentHash

Validation

• original relevant .NET suites: 99 passed
• final preview suite: 8/8
• adjacent atomic-batch suite: 24/24
• DS462 repeated isolation stress: 25/25
• final complete stacked solution suite: 3,708 passed, 3 skipped, 0 failed
• Python atomic-batch tests: 4 passed
• npm typecheck passed
• Release WASM publish passed
• real Chromium batch tests: 4 passed
• worktree and diff checks clean

Stacked on the atomic-batch foundation in #479.

Closes #446