`x(0) == v` with `v` the variable reports `UndefVarError: `v##NNNN` not defined in `Main`` instead of a clear message

[ocots]

Context

Found while making OptimalControl's docs/src/modelling/abstract-syntax.md executable
(control-toolbox/OptimalControl.jl#913). The page documents the rule "constraint bounds
must be effective, that is must not depend on a variable" and shows a rejected example.
The example is rejected — but with an internal-gensym UndefVarError that tells the
reader nothing about the cause.

Reproduce

using OptimalControl   # CTParser 0.9.4-beta, CTBase 0.30.2-beta, CTModels 0.19.2-beta

@def begin
    v ∈ R, variable
    t ∈ [0, 1], time
    x ∈ R², state
    u ∈ R, control
    -1 ≤ v ≤ 1
    x₁(0) == -1
    x₂(0) == v            # a boundary "bound" that references the variable
    x(1) == [0, 0]
    ẋ(t) == [x₂(t), u(t)]
    ∫(0.5u(t)^2) → min
end

Result:

Line 7: x₂(0) == v
UndefVarError: `v##3260` not defined in `Main`
Suggestion: check for spelling errors or missing imports.

The v##3260 is a leaked internal gensym for the variable vector; it is referenced in
generated boundary-constraint code but is not in scope there.

Expected

A ParsingError (or similar) naming the real cause — e.g. "a constraint bound may not
depend on the variable v; write it as a boundary constraint instead, e.g. x₂(0) - v == 0".
The working form x₂(0) - v == 0 builds fine.

Impact

Low severity (the construct is rejected, just unhelpfully), but the message is user-facing
and currently misleading. OptimalControl#913 works around it by softening the doc prose;
this issue tracks the real fix.

[ocots]

Fix in #345 (stacked on #326, targeting chore/deps-examodels-0.12).

p_constraint! now rejects a constraint whose bound depends on v, the state, the control or the time, with a clear ParsingError:

the lower bound of a constraint must not depend on the variable; write a functional
constraint instead by moving the term to the constrained side (e.g. `x₂(0) - v == 0`
rather than `x₂(0) == v`)

The check runs before the :fun / :exa dispatch, so v == x₂(0) and x₁(0) ≤ x₂(0) are caught as well. The constrained side may still reference the variable (e.g. 0 ≤ r(0) - z ≤ 1); only the bounds are restricted. x₂(0) - v == 0 keeps building fine.