ci: baseline semver-checks against origin/main instead of crates.io

[rcoh]

Switch the Semver checks CI job to use --baseline-rev origin/main instead of the default crates.io baseline.

Why

By default, cargo-semver-checks compares the PR against the latest crates.io release. That's good for "must I bump major before publishing?" but noisy for per-PR review:

• Every PR sees every accumulated breaking change since the last release, drowning out what this PR introduced.
• Stacked PRs relitigate the same advisory warnings.

--baseline-rev origin/main makes the per-PR check report only what the PR itself introduces. The "must I bump major?" check is already separately handled at release time by release-plz's own semver_check = true (per CONTRIBUTING.md), so we don't lose any release safety.

Changes

• actions/checkout: fetch-depth: 0 (need full history for the baseline rev).
• cargo semver-checks: add --baseline-rev origin/main.

Reference: https://github.com/obi1kenobi/cargo-semver-checks#does-the-crate-im-checking-have-to-be-published-on-cratesio

Expected effect on this PR

This PR itself touches only .github/workflows/ci.yml, so the new semver-checks job has nothing to compare and should pass.