[mvanhorn]

Closes #486.

fs.stat already reports mode. Readers that use only the Store still had to
call it once per directory entry for times and link counts, and it did not
report either. This adds those fields end to end.

What changes

• fs.stat reports mtimeSeconds, mtimeNanoseconds and nlink beside
  type, inode, size and mode.
• fs.list entries report mode, mtimeSeconds, mtimeNanoseconds and
  nlink beside name, path, type and inode.
• nlink is the committed namespace link count: 0 for the root, 1 for any
  other directory or symlink, and the number of names for a regular file.
  Hard-linked names share inode.
• mtimeSeconds / mtimeNanoseconds are the portable modification time
  (Unix epoch seconds and the fractional second). They are two integers so a
  JavaScript Number keeps them exactly. A single nanosecond timestamp is not a
  safe integer after 1970.
• Inline portable metadata is still served from the inode value (no object
  read). Root-form metadata uses the listing's existing memo.
• The listing reply budget per entry is 240 bytes to cover the extra fields.
• CLI summaries for fs stat and fs list name the new fields.
• Docs: api/docs/agent.md covers the fields and the listing.

No Store format change: every entry already stores mode, mtime and the
namespace link count. inode was already the hard-link group id.

Commits

• feature, tests, docs and golden. Production LOC: 102831 -> 102851 (delta +20).

Tests

No test deleted or weakened. The sdk_methods golden adds the new fields to
its fs.stat and fs.list lines; mtime integers are masked as <varies>.

• New engine/crates/layerfs-api/tests/fs_stat_metadata.rs: Init keeps a
  host mtime and hard link; fs.stat and fs.list agree; a content write's
  mtime reads back; main is untouched.
• sdk_method_golden, sdk_list_entries, fs_mode, stat_symlink,
  write_refusals: pass.
• rustfmt, and Clippy -D warnings on layerfs-store-service and
  layerfs-api: clean.
• JS against the fake binding: api/agent/test/calls.mjs and
  api/cli/test/cli.test.mjs: pass.

Not run: the macOS/Windows worker routes. They forward the same SDK calls and
were not changed.

Author

• I ran the checks listed above
• Docs match the implemented fields

Demo

[Mola-maker]

感谢贡献，也感谢 AI 协作工具在 #486 的讨论中梳理接口面 🙏

真实评估（已读 diff）：fs.stat 增加 mtimeSeconds/mtimeNanoseconds/nlink，fs.list 条目带上同组字段。两个设计决定都对：(1) 秒/纳秒拆成两个整数，避免 JS Number 的精度丢失（>2^53 纳秒时间戳会丢精度）——这正是 #486 讨论里的关键点；(2) nlink 取"已提交命名空间的硬链接名计数"（root 为 0、目录/symlink 为 1），语义写进了 api/docs/agent.md 与新增的 fs_stat_metadata.rs（156 行测试）。硬链接名共享 inode 的既有契约保持。没有发现缺陷。

对 AI agent 的使用价值：agent 遍历大目录（如 node_modules、数据集）时不再需要每个 entry 补一次 fs.stat，扫描类工具调用量直接下降。

建议：合入前在真实环境对含硬链接的树实测一次 fs.list 输出，确认 nlink 与 inode 配对正确。功能性调整等 LayerFS 2.0（#446）发布后再做。