refactor: Higgs GaugeGroupI action to a Representation

[doxtor6]

Summary

Resolves the TODO in HiggsBoson/Basic.lean: the action of GaugeGroupI on HiggsVec is
now a directly-defined Representation ℂ GaugeGroupI HiggsVec (repGaugeGroupI), and the
SMul/MulAction/DistribMulAction/SMulCommClass instance stack is removed, so the
representation is the single spelling of the action on HiggsVec.

• Lemmas restated in rep form: repGaugeGroupI_apply (and the U1/SU2 variants),
  repGaugeGroupI_inner/_norm, toRealGroupElem_apply_self,
  ofU1Subgroup_repGaugeGroupI_apply, repGaugeGroupI_phase_snd; the orbit lemma becomes
  exists_repGaugeGroupI_eq_iff_norm_eq (stated with an explicit existential).
• New group-property lemmas (repGaugeGroupI_mul_apply, inverse cancellation simp lemmas,
  repGaugeGroupI_inv_apply_eq_iff) replace mul_smul/inv_smul_smul downstream.
• EffectivePotential: IsInvariant is stated via the representation; the smul_comm step
  is replaced by linearity (LinearMap.map_smul_of_tower).
• TwoHDM: TwoHiggsDoublet keeps its MulAction (its orbit API needs it), now defined
  componentwise through repGaugeGroupI; GramMatrix orbit lemmas ported to the rep form.

Carved out of #1415, where the agreed convention is that gauge actions on field target spaces
are Representations rather than MulAction instances; this part is independent of the EFT
algebra work and useful on its own. The corresponding EFTLagrangianExclDeriv hookups stay
in #1415, since those files only exist there.

Built locally (the four changed files and their two downstream importers,
HiggsBoson/Potential and TwoHDM/Potential).

🤖 Generated with Claude Code

[github-actions]

Thank you for this pull-request (PR). If this is your first PR, welcome to the community!

Below is what will happen next. Please read carefully if you are not familiar with the process. You may open other PRs while this one is being reviewed, and can stack PRs on top of each other, so don't let these steps slow you down.

1. Some automated checks will be run on your PR. You can see the results of these checks at the buttom of your PR page. If any of these checks fail, you will need to fix the issues before your PR can be merged. You can learn more about these here, including how to run them locally, which is sometimes quicker than relying on the GitHub Actions. If you have never had a PR merged before, you may have to wait for a reviewer to manually start these checks (this is for security).

2. A reviewer will look at your PR and may ask you to make changes. This may happen a couple of days after you submit your PR, so you may need to be patient. But it should not be longer than that - if it is please bring it to the attention of the community on the Zulip. The level of review will depend on where your PR is submitted. If it is submitted to ./Physlib or ./QuantumInfo, the review will be more thorough than if it is submitted to ./PhyslibAlpha. You can find out more about what the review process is looking for in our review guidelines. If a reviewer adds an awaiting-author label to your PR, address the review comments, then please remove that label by adding a comment with -awaiting-author. This helps us keep track of reviews.

3. The reviewer will either approve your PR, or request more changes (in which case we return to step 2). Once your PR is approved, it will be merged by a maintainer, this should happen shortly after approval, though you may get more comments at this stage.

Tip: The easiest way to get have a fast review is to submit a PR that is small and self-contained, and has clear documentation explaining why things are the way they are in your chages.

If you have any problems or questions, please reach out to the community on the Zulip.

[jstoobysmith]

Looks good to me! @nateabr do you also want to take a look?

[nateabr]

Yep looks great to me!