feat(git): materialize exact Gitoxide projections

[zhiiw]

Summary

This stacked Draft completes the first Gitoxide workspace data plane with a fresh, exact projection boundary:

Only an owner-bound managed-repository capability may materialize its exact accepted commit into a previously absent staging directory, and only the resulting projection capability may observe whether every path, type, byte, and supported mode still matches the immutable Git tree.

What this proves

• repository path, accepted commit, and tree remain capability-bound rather than caller-asserted;
• projection roots use a fresh-directory boundary and files use create-new semantics;
• the helper writes no .git metadata and creates no linked worktree/control capability;
• source constraints and bounded file/tree budgets are enforced again during materialization;
• observation detects missing, extra, type-changed, same-size content-drifted, and POSIX executable-mode-drifted paths;
• regular files are opened with no-follow/reparse-point semantics during observation;
• response loss converges only when an existing destination is still an exact projection of the accepted tree; a partial/drifted directory is never overwritten;
• the helper never deletes a projection. Private staging cleanup, rotation, and quarantine remain composition-owner responsibilities.

Deliberate exclusions

No Desktop/CLI consumer, canonical projection rotation, quarantine GC, SQLite accepted head, or Write/Edit recovery is added here. Power-loss durability is not claimed. M1.3 product composition is the next slice; M2.2/M2.4 stay deferred until that composition consumes this data plane.

Verification

• Core, Storage, Runtime, and Runtime Host builds pass locally.
• The parent candidate/ref CAS PR is green on Linux, macOS, and Windows.
• This PR uses the same three-platform workflow for Rust formatting/tests and real JavaScript helper authority tests.

Depends on #18. Keep Draft until all platform checks pass and its parent lands.

中文说明

这个堆叠 Draft 补齐第一版 Gitoxide workspace 数据面的 fresh projection 边界：只有 owner-bound managed-repository capability 能把绑定的 exact accepted commit 物化到此前不存在的 staging 目录；只有随结果签发的 projection capability 能观察该目录是否仍与 immutable Git tree 完全一致。

helper 不写 .git、不创建 linked worktree，也不删除任何 projection。observer 会检查缺失/额外路径、类型、bounded content hash，以及 POSIX executable bit，并以 no-follow/reparse-point 方式打开普通文件。响应丢失后，只有现存目录仍是 exact projection 才返回同一成功结果；partial/drifted 目录不会被覆盖。

本 PR 不做 Desktop/CLI 接线、canonical rotation、quarantine GC、SQLite accepted head 或 Write/Edit recovery。下一步是 M1.3 product composition；M2.2/M2.4 继续等待数据面与产品接线完成。

[zhiiw]

Closed as an obsolete extraction/integration slice. The current delivery path is the merged Gitoxide foundation followed by #40 -> #41 -> #42. This PR remains available as design and implementation history.

中文说明

该 PR 属于旧 extraction/integration 栈，现已失去独立合并价值。当前交付路径是已合并的 Gitoxide 基础，然后依次审查 #40 → #41 → #42。这里继续保留设计、实现与审查历史。