Feature/extend velocity along normals

[Fastaxx]

Related issues

#70

Summary

This PR adds normal-velocity extension for interface-defined scalar speeds and a small API improvement to NormalMotionTerm.

• NormalMotionTerm now accepts an optional update callback (like AdvectionTerm) so a mesh-based speed field can be mutated before each RK stage:

vfield = MeshField(x -> 0.0, grid)
term = NormalMotionTerm(vfield, (v, ϕ, t) -> (values(v) .= 0.25 + 0.1 * t))
term

• Adds extend_along_normals! to extend an interface-only speed (common in Stefan problems where (v_n) is known only near ϕ=0) into a narrow band, then use it with NormalMotionTerm:

ϕext = LevelSetMethods.star(grid)
v = zeros(Float64, size(grid)...)
Δ = minimum(LevelSetMethods.meshsize(grid))
frozen = abs.(values(ϕext)) .<= 1.5Δ
for I in CartesianIndices(v)
    frozen[I] || continue
    x = grid[I]
    v[I] = 0.2 + 0.1 * cos(2π * atan(x[2], x[1]))
end
extend_along_normals!(v, ϕext; frozen, nb_iters = 80)
term = NormalMotionTerm(MeshField(v, grid, nothing))
term

Checklist

• I am following the [contributing guidelines](https://github.com/maltezfaria/LevelSetMethods.jl/blob/main/docs/src/90-contributing.md)
• Tests are passing
• Lint workflow is passing
• Docs were updated and workflow is passing

[codecov]

Codecov Report

❌ Patch coverage is 95.94595% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.34%. Comparing base (f69cfcb) to head (b1af1cc).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/velocityextension.jl	97.01%	2 Missing ⚠️
src/levelsetterms.jl	85.71%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (f69cfcb) and HEAD (b1af1cc). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (f69cfcb)	HEAD (b1af1cc)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #73       +/-   ##
===========================================
- Coverage   78.16%   50.34%   -27.82%     
===========================================
  Files          14       15        +1     
  Lines         948     1007       +59     
===========================================
- Hits          741      507      -234     
- Misses        207      500      +293     

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