docs(tvix/store): add README, document services and store model

These are intended to help digest the protocol definitions for tvix-
store, and how they tie into the whole concept.

Co-Authored-By: Vincent Ambo <mail@tazj.in>
Change-Id: Ic1ba3ba41ef599209453f15d0ac2e07a6144bcca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7439
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/docs/castore.md

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+# //tvix/store/docs/castore.md
+
+This provides some more notes on the fields used in castore.proto.
+
+It's meant to supplement `//tvix/store/docs/api.md`.
+
+## Directory message
+`Directory` messages use the blake3 hash of their canonical protobuf
+serialization as its identifier.
+
+A `Directory` message contains three lists, `directories`, `files` and
+`symlinks`, holding `DirectoryNode`, `FileNode` and `SymlinkNode` messages
+respectively. They describe all the direct child elements that are contained in
+a directory.
+
+All three message types have a `name` field, specifying the (base)name of the
+element (which MUST not contain slashes or null bytes, and MUST not be '.' or '..').
+For reproducibility reasons, the lists MUST be sorted by that name and also
+MUST be unique across all three lists.
+
+In addition to the `name` field, the various *Node messages have the following
+fields:
+
+## DirectoryNode
+A `DirectoryNode` message represents a child directory.
+
+It has a `digest` field, which points to the identifier of another `Directory`
+message, making a `Directory` a merkle tree (or strictly speaking, a graph, as
+two elements pointing to a child directory with the same contents would point
+to the same `Directory` message.
+
+There's also a `size` field, containing the (total) number of all child
+elements in the referenced `Directory`, which helps for inode calculation.
+
+## FileNode
+A `FileNode` message represents a child (regular) file.
+
+Its `digest` field contains the blake3 hash of the file contents. It can be
+looked up in the `BlobService`.
+
+The `size` field contains the size of the blob the `digest` field refers to.
+
+The `executable` field specifies whether the file should be marked as
+executable or not.
+
+## SymlinkNode
+A `SymlinkNode` message represents a child symlink.
+
+In addition to the `name` field, the only additional field is the `target`,
+which is a string containing the target of the symlink.