feat: add did:plc parse method

[alanshaw]

Adds a parse method for did:plc DIDs.

The DID itself is derived from the hash of the first operation in the log, called the "genesis" operation. The signed operation is encoded in DAG-CBOR; the bytes are hashed with SHA-256; the hash bytes are base32-encoded (not hex encoded) as a string; and that string is truncated to 24 chars to yield the "identifier" segment of the DID.

In pseudo-code: did:plc:${base32Encode(sha256(createOp)).slice(0,24)}

https://github.com/did-method-plc/did-method-plc/blob/main/website/spec/v0.1/did-plc.md

[Copilot]

Pull request overview

Adds first-class parsing/validation for did:plc identifiers in the PLC DID package, making it easy to safely accept external DID strings (not just ones generated by plc.New) while centralizing the identifier-length invariant.

Changes:

• Introduces plc.Parse to parse a DID string, enforce the plc method, enforce a 24-character identifier, and validate base32-lower/no-padding encoding.
• Adds IdentifierLength constant and uses it when slicing the generated identifier in plc.New.
• Adds comprehensive unit tests covering valid parsing and common invalid inputs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
did/plc/plc.go	Adds Parse and a shared IdentifierLength constant; updates New to use the constant.
did/plc/plc_test.go	Adds TestParse to validate acceptance/rejection behavior for plc.Parse.

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