Fix the Documentation build under ModelingToolkit v11

[ChrisRackauckas-Claude]

Summary

The Documentation workflow has been red since MTK v11 became resolvable in docs/Project.toml (the v9 cap from #64 was undone by the dependency bump in #65). Rather than capping again, this migrates the lecture examples to the current APIs. Root causes, in order of how many blocks they broke:

• The @mtkmodel/@connector component DSL now lives in SciCompDSL.jl, so lecture 1 needs using SciCompDSL.
• DifferentialEquations v8 no longer re-exports the solvers and initialization algorithms used here, so NewtonRaphson, DFBDF, ImplicitEuler, ShampineCollocationInit and NLNewton need NonlinearSolve, OrdinaryDiffEqBDF, OrdinaryDiffEqSDIRK and OrdinaryDiffEqNonlinearSolve.
• ModelingToolkit.missing_variable_defaults and ModelingToolkit.defaults were removed; problems now take a symbolic operating-point map, and defaults are split into bindings and initial_conditions.
• A symbolic default such as @variables x(t) = x₀ is now a binding, which cannot be overridden by a later u0. Where the lectures deliberately override those states (restarting from a previous solve), the values are passed via the initial_conditions keyword instead.
• MTKStandardLibrary constructor changes: Mass no longer takes v, MassFlow no longer takes p_int, Open takes p, and HydraulicPort no longer takes p_int.
• sol.destats is now sol.stats, and integer indexing of a solution (sol[2]) is element-wise, so the state after one step is sol.u[2].
• Flattened variable names containing ₊ (the namespace separator) silently lose their defaults, so the hydraulic debugging model in lecture 6 uses _ instead. That model is also rewritten in the functional ODESystem style already used by the rest of the lecture, since @mtkmodel System now collides with ModelingToolkit.System.

The Volume component that lecture 6 already carried locally is now also used by lecture 2, whose IsothermalCompressible.Volume usage no longer matches the library version.

Test plan

• julia --project=docs docs/make.jl equivalent run locally (Documenter 1.17, MTK v11.40, MTKStandardLibrary v2.29.7): all pages expand, no @example/@repl failures and no Documenter errors. linkcheck was disabled locally since the sandbox has no outbound network; CI still runs it.
• Pkg.resolve() on docs/ succeeds with the updated compat bounds.

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