FE-1517: Scenario creation authors through the ad-hoc form

[kube]

Stacked on #9290 (FE-1490). Part of the ad-hoc scenario stack (FE-778).

Behind the Ad-hoc scenarios setting, the scenario creation form authors scenarios through the ad-hoc Initial State + Parameters form, in a new expose selection mode:

• Each top-level Variable row carries a Scenario Parameter toggle (AdHocVariable.exposed); exposed Variables become the saved scenario's tunable parameters (snake_cased identifier, constant expression as the default).
• One mode only: no "Define as code" toggle on this surface.
• The definition persists as initialState: { type: "adhoc", content }; scenarioParameters and parameterOverrides are derived from it on save and stay the compiler's inputs, so every other consumer of the saved scenario works unchanged.
• The consolidated form spec (content/ui/ad-hoc-form.mdx) gains a persisted-format section: the initialState variants, the AdHocScenarioState tree, the derived-on-save fields, and the compile-time-only code.
• The view/edit drawer round-trips the ad-hoc variant — always through the ad-hoc form, whatever the setting says, since the classical form cannot represent it.
• Lowering a persisted ad-hoc scenario passes the net context through the language worker (sdcpn/lowerScenario now accepts adHocContext), the CLI, and useScenarioHir.
• Drawer footers count the ad-hoc form's own LSP diagnostics (session-scoped, so a second mounted form never leaks its errors in).

User guide, architecture spec, and changeset included.

🤖 Generated with Claude Code

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