docs(specs): align ceremony browser contract

[Wondertan]

Summary

This PR aligns the normative ceremony rules with the browser protocol. It targets main directly and keeps the existing terminology so the protocol changes can merge independently.

It supersedes #15, which GitHub marked merged into the terminology branch while the stack order was being reversed.

Changes

Ceremony boundary and lifecycle

• Define the Ceremony as the off-ledger process that authenticates the selected identity-platform account, derives its canonical user ID and handle, and locally generates the exact OAuth Proof.
• Retain one exact platform profile for a live ceremony and forbid profile substitution after authorization starts.
• Keep ceremonies one-shot: remove normative mid-ceremony persistence and resume, and require a fresh GitHub ceremony when a consumed exchange response is lost.
• Assign browsing contexts, redirect transport, interruption behavior, and application handoff to the browser architecture.

Proof construction and ledger selection

• Rename platformVerifierVersion to platformCeremonyVersion and define it as the shared version boundary for the Authorization Digest construction, platform OAuth construction, and platform-specific proof statement.
• Keep verifier contracts and implementations outside that boundary: different ledgers may enforce the same Platform Ceremony Version with different verifier artifacts.
• Remove the separate pkceNonce; derive the PKCE verifier from the Authorization Digest and its existing fresh authorizationNonce.
• Keep that nonce private until token exchange completes, then reuse the copy already carried by the OAuth Proof for verifier recomputation.
• Remove the redundant PKCE domain and update the golden vector and conformance requirements.
• Let application composition select a Ledger Profile for each ceremony while requiring the Verifier Dispatcher to derive the Ledger ID independently from the Execution Ledger.

Authenticated delivery

• Permit a deployment-configured set of application origins.
• Deliver only over a live channel authenticated to an exact configured origin.
• Ignore caller-supplied forwarding targets and add multi-origin positive and foreign-origin negative coverage.

Platform behavior

• Define the GitHub Token Service's semantic inputs, outputs, statelessness, fixed configuration, and isolated-prover boundary while leaving endpoint and wire formatting to browser/deployment specifications.
• Preserve the local checks binding the returned attestation, authorization code, client, redirect, verifier, bearer opening, and disclosure.
• State directly that Google has no server-side token exchange; X remains browser-owned.
• Require X's first notarized token request to reach X within the 30-second authorization-code deadline; later response and proving work are outside it.

Scope

Normative documentation only:

• specs/libid.md
• specs/ceremony-common.md
• specs/platform-ceremonies.md

The TypeScript ceremony architecture and package test plan remain in #13.

Validation

• git diff --check