Fix browser signer discovery

[kalepail]

What changed

• Use the portable base64url helper for Mercury passkey credential IDs.
• Add a regression test with the installed browser Buffer polyfill.
• Prepare passkey-kit 0.16.2 and update the release notes.

Why

Browsers use buffer 6.0.3. It does not support Buffer.toString with the base64url encoding. Signer discovery therefore failed with Unknown encoding: base64url.

Impact

Browser applications can discover passkey signers again. No contract, binding, indexer, or relayer change is required.

Closes #2.

Checks

• pnpm test --run: 216 tests passed
• pnpm run verify:bindings: passed
• pnpm run build: passed
• pnpm run build:demo: passed
• Claude Fable independent review: approved

[Copilot]

Pull request overview

Fixes browser signer discovery by using portable base64url encoding and prepares release 0.16.2.

Changes:

• Replaces unsupported browser Buffer encoding.
• Adds browser-polyfill regression coverage.
• Updates package version and release documentation.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/indexer/mercury.ts	Uses the portable base64url helper.
src/indexer/mercury.test.ts	Tests discovery with browser Buffer.
src/version.ts	Updates generated version.
package.json	Bumps package to 0.16.2.
docs/releasing.md	Updates checked-in release versions.
CHANGELOG.md	Documents the browser fix.

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