Spec 5.1: flow-casfs serves text files as renderings cached by graph key (refs #21) 701dcbc5d468

2. The state's tree is read from a state index (Oxygen's flat, checksummed

   description of a history-only state: its changes, tree view and each

   file's deferred history with its graph key), built once from the

   nearest checkpoint plus the changes after it and kept beside the store.

   Listing a directory needs nothing else.

3. The tree is exposed to the sandbox read-only through flow-casfs (FUSE):

   - An opaque file's bytes are its stored blob, whose chunks (FastCDC,

     32 KiB to 256 KiB, mean 64 KiB, Oxygen spec Section 13.2) are read as

     they are needed.

   - A text file's bytes are a rendering of its history, not a stored blob.

     Renderings are cached by **graph key** (equal keys mean equal

     renderings) as ordinary Blob and Chunk objects: a file is rendered on

     first read or stat, and every later state in which it has the same

     history finds it at once. A file added once and never edited is its

     own text blob and needs no rendering at all.

   - Missing objects are fetched lazily using the POST /objects protocol.

   - To prevent FUSE context-switch bottlenecks on many small reads,

     flow-casfs caches whole small files and mmaps chunk data, relying on

     the host page cache.

   - All guest mutations are diverted to an overlay on the job's ephemeral

     disk (or a tmpfs); base objects remain immutable.

4. The same mount backs web workspaces (OxygenHub spec 13.3): a

   `workspace` job keeps it for as long as its editor is open.

5. Targets, measured on the Linux tip: mount under 100 ms, a first read of

   a file in milliseconds, and a full read of the tree no slower than a

   checkout writing it.