goat: repo inspect --mst flag for displaying MST structure

[devinivy]

This adds a flag --mst to goat's repo inspect command. When passed, the command will output a representation of the repository's MST structure. It may also be used to display the partial MST of a repository, i.e. in the case of a record proof.

As part of this work I exposed the NodeData and TreeEntry types from the mst module, which are sufficient for working with raw MST data.

The output is intended to be a faithful representation of the MST:

• Siblings items are entries within a single data node.
• The dots (....) represent the length of the key prefix.
• The text after the dots is the rest of the key.
• The hashes in parens are the last 7 characters of the CID pointer represented in base32.
• Each CID pointer points to either a. a subtree (i.e. another data node) when they appear alone or b. a record block when they appear next to a key.
• The ✗ appears in the output when the block behind a CID pointer is not present, which should not occur when inspecting a full repository but is generally expected when inspecting a proof.

Example:

$ curl 'https://morel.us-east.host.bsky.network/xrpc/com.atproto.sync.getRecord?did=did:plc:l3rouwludahu3ui3bt66mfvj&collection=app.bsky.feed.like&rkey=3jwnnk25rrk2s' > record-proof.car
$ goat repo inspect --mst ./record-proof.car
(lxmb3rm)
├── (cpibfny)
│   ├── (veqbjjq)
│   │   └── (s4zluvy)
│   │       ├── (6qiyhle) ✗
│   │       ├── app.bsky.feed.like/3jqa6sh7bic2a (edo6qsa) ✗
│   │       ├── (xswayqm) ✗
│   │       ├── .....................sfpt3arg22b (glnh3mi) ✗
│   │       ├── (55dhu6q) ✗
│   │       ├── .....................urpqzpats2e (xqygjyq) ✗
│   │       ├── (pblbwl4) ✗
│   │       ├── .....................wnnk25rrk2s (feoom4e)
│   │       └── (3jwkksm) ✗
│   ├── app.bsky.feed.like/3kb4ztebs4324 (hc4ruf4) ✗
│   └── (t4os5oq) ✗
├── app.bsky.feed.post/3kps37ajdyk24 (yk4ymya) ✗
└── (chuh4na) ✗

[bnewbold]

I like it! And cool how (relatively) little code this is.

A couple high-level notes before I dig in to code and play with it myself:

• I think this could be a separate command: goat repo mst
• would be very open to color for this. I don't think we have a CLI color library in go but would be open to adding a small one as a dep
• the partial CIDs make me double-take. what if we prefixed like cid:lxmb3rm? or maybe in square brackets with elipses [lxmb3rm…]? color might help. maybe a CLI flag/arg to control "full CID" vs "partial CID" (for those of us with big monitors?)
• I like visually differentiating the partial "key" structure, but don't like the collision that technically a period could appear in MST text (even if that would be invalid at the "atproto repo" layer). we could print the full key (collection/rkey) and use "grey text" vs "bold white" to visually emphasize?

Additional feature brainstorming:

• any special display for the "commit" object as a node above the "root MST" node?
• ability to visualize entire tree, but highlight the "proof chain" to a single record (by collection/rkey path)
• ability to parse a full repo CAR file, but display only the proof chain to a single record (distinct from highlighting within the full tree structure)
• some kind of JSON output structure