docs(specs): align ceremony browser contract

[Wondertan]

Summary

This PR aligns the normative ceremony rules with the browser protocol. It is stacked on the terminology-only PR and contains the behavior changes only.

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

• 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 and its fixed /oauth/github/token request, response, origin, size, media-type, cache, and statelessness rules in the GitHub ceremony.
• 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

• Final three-file tree is byte-identical to the prior docs(specs): align ceremony browser contract #15 tip.
• git diff --check

[Wondertan]

The stack was intentionally reversed so protocol behavior can merge before terminology. GitHub marked this PR merged when its rewritten head became an ancestor of the then-base terminology branch, so the behavior-only change now continues in #20; #19 is stacked on top for terminology review.