docs(modelling): make abstract-syntax.md executable (Phase E1)

[ocots]

What

docs/src/modelling/abstract-syntax.md — the most visited page of the site — described the
whole @def grammar but nothing on it was verified by the build: 0 @example, 41 inert
```julia fences (the campaign count of 37 missed 4 fences indented inside !!! warning
admonitions), 4 `@repl`. Every fragment could be wrong and CI would stay green. The blocker
was mechanical: the `...` idiom inherited from `docs/attic/` left every `@def` incomplete,
and `@def` validates at build time.

This is Phase E1 of the documentation campaign.

Changes (documentation only — no API change)

• 30 non-notation fences → executed blocks: 27 @example + 7 @repl, all sharing one
  @setup abs module (using OptimalControl + two stubs, data(t) and c(t)).
  • single-clause blocks: complete @def, the off-topic clauses carry # hide, block ends
    nothing # hide — the reader sees only the taught clause.
  • whole-problem blocks (dynamics variants, constraints showcase, Bolza): shown in full.
  • error cases: @repl + try # hide / catch e # hide / showerror(IOContext(stdout, :color => false), e) # hide / end # hide — renders a clean REPL[1] box, no
    stacktrace, no path leak.
• 11 :( … ) grammar patterns stay inert, covered by one opening !!! note — they are
  Expr templates with $-placeholders, not runnable code.
• Three latent bugs fixed (the inert fences hid them; each verified at the REPL):

  was	result	fix
  ∫(x(t) - data(t))² → min	Julia ParseError: identifier cannot begin with character '²'	∫((x(t) - data(t))^2) → min
  -ω²*q(t), ω² → min (ω is 1-D)	@def keeps ω² as a free undefined symbol; breaks at solve	-ω^2*q(t), ω^2 → min
  u ∈ R², control + c(t) * u(t)^2	integrand eval → MethodError: no method matching ^(::Vector{Float64}, ::Int64)	u ∈ R, control across the Bolza section (matches the Lagrange section above)

• Voice: her / the user → you / your (4 sites); fixed a copy-paste "state" → "control"
  in the control-aliases sentence.

Verification

Full docs build (julia --project=. docs/make.jl) run twice, exit 0, no failed to run,
every @def on the page executed. The only warnings are the pre-existing warnonly
external-link ones (Phase D backlog). Add the run documentation label to build the site in
CI.

Follow-ups (recorded in .reports/campaign/)

• Two CTParser rough edges surfaced and are being filed as issues: x(0) == v (bound
  depending on the variable) reports a leaked-gensym UndefVarError instead of a clear
  message; @def … end true trace mode prints the parsed model twice.
• New Phase J on the campaign board: review where the docs could use the unused
  VitePress code features ([!code ++/--/highlight], focus, code groups) — static fences
  only, since they do not work inside executed @example blocks.

🤖 Generated with Claude Code