auto-task(StandardModel.Representations): record TODO for Standard Model fermion content as left-handed Weyl fermions

[jstoobysmith]

Summary

Records a single TODO in the Standard Model representations file for the structure
holding the Standard Model's fermionic content, expressed entirely as left-handed
Weyl fermions.

The TODO was written by a human (the task author); Claude placed it in the
correct file and section of the library and verified the build and linters.

Original request

• What TODO item would you like to add to the Physlib repository?
  The structure containing the fermionic content of the Standard Model, all as
  left-handed Weyl fermions. The structure should contain an instance of a module,
  the Lorentz group action, the action of the global Gauge group of the Standard
  model. It should contain all three families.
• Name (for copyright header), if needed: Joseph Tooby-Smith

What changed

In Physlib/Particles/StandardModel/Representations.lean, added:

TODO "Define a structure capturing the fermionic content of the Standard Model, with all fermions
  expressed as left-handed Weyl fermions (`Fermion.LeftHandedWeyl`) and including all three
  families. The structure should carry a `Module ℂ` instance together with a representation of the
  Lorentz group and a representation of the global gauge group `GaugeGroupI` (built from `repU1`
  and `fundamentalSU2`)."

The required public import Physlib.Meta.TODO.Basic was added, since this file only
imported pure Mathlib and the TODO command did not otherwise elaborate.

Why this location

Representations.lean defines the representations appearing in the Standard Model
(repU1, fundamentalSU2) and lives alongside Basic.lean which defines the global
gauge group GaugeGroupI. The fermion-content structure is exactly the matter that
transforms in these representations, so this is the most specific existing home. The
TODO text refers to the relevant existing definitions (Fermion.LeftHandedWeyl,
GaugeGroupI, repU1, fundamentalSU2) so a future contributor has the building
blocks to hand. No equivalent TODO already existed.

Verification

• lake build succeeds with no new errors, warnings, sorrys, or axioms.
• lake exe runPhyslibLinters — clean.
• ./scripts/lint-style.sh — clean.

No declarations were implemented or proved; this PR only records the TODO.

---

Human review

• Does the TODO item look correctly placed in the library and an accurate reflection of the input? Yes

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