Add RUSTSEC-2026-0173 exception

[RobertZ2011]

aquamarine depends on proc-macro-error2 which is now unmaintained.
Remove it as it provides a minor QoL improvement. Mermaid diagrams are
still present in the docs directory and Github supports rendered
previews. Add an exception since defmt also depends on this crate.

[github-actions]

Cargo Vet Audit Passed

cargo vet has passed in this PR. No new unvetted dependencies were found.

[Copilot]

Pull request overview

This PR removes the aquamarine dependency and stops embedding rendered Mermaid diagrams into rustdoc, keeping the Mermaid source files under docs/ for GitHub-rendered previews. It also updates dependency/security configuration to account for the resulting dependency graph changes.

Changes:

• Remove aquamarine from dependencies and from the lockfile.
• Remove rustdoc attributes that embedded the Mermaid state machine diagram in StateMachine docs.
• Add a cargo-deny advisory ignore entry for proc-macro-error2.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/ucsi/ppm/state_machine.rs	Removes rustdoc Mermaid embedding attributes for the PPM state machine docs.
deny.toml	Adds an ignore entry for RUSTSEC-2026-0173 (proc-macro-error2).
Cargo.toml	Drops the aquamarine dependency.
Cargo.lock	Removes aquamarine and its transitive deps from the resolved dependency set.

[kurtjd]

If we still need the exception for defmt, does it make sense to keep aquamarine around anyway? Or do you foresee defmt moving off the dependency but not aquamarine?

[RobertZ2011]

If we still need the exception for defmt, does it make sense to keep aquamarine around anyway? Or do you foresee defmt moving off the dependency but not aquamarine?

It's more that defmt is a requirement while aquamarine isn't. But I also felt that it ended up being more of a gimmick and there's some weirdness about vetting since it has to be pulled in as a run-time dependency to run during cargo docs.