docs(higgs): add Higgs field API map

[Robby955]

Motivation

Related to #1414 and the tracking issue #857, which is still an unfilled template, so the map supplies the requirement list it asks for. The Two Higgs Doublet Model map already lists Higgs Field (Physlib/Particles/StandardModel/HiggsBoson) as a parent API, so this map also fills a node that an existing map references. An API map records the implemented status and source location of each requirement, so the gap between a planned API and the current library stays visible in-tree.

Changes

Adds Physlib/Particles/StandardModel/HiggsBoson/API-map.yaml, with 21 requirements, 16 of them done. The completed entries cover HiggsVec with its basis, real-scalar coordinates, and the repGaugeGroupI representation with its invariance and orbit-by-norm theory; HiggsBundle and HiggsField with constant fields and component smoothness; the pointwise inner product and norm-squared with their algebra and smoothness; the Potential structure with its completing-the-square, discriminant, boundedness, and minimisation theory; and EffectivePotential with gauge invariance, factorisation through the norm, and the polynomial expansion in powers of the norm.

Five are recorded as not done: the stability-group lemmas and the global gauge action on HiggsField exist only as informal declarations, and the map cites them as such; the Higgs bundle is still the trivial bundle rather than an associated bundle; effective potentials are not yet connected to field-level potentials; and covariant derivatives with kinetic terms wait on a gauge-connection API. Nothing outside the new file changes, and no Lean source is touched.

Checks

python3 scripts/api_map_linter.py --repo . passes with no missing files and no missing names, resolving 91 declarations in the new map against the Lean sources.

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[jstoobysmith]

Approved - looks good to me. Thanks