UI: grey ribbon entries, search results and Solve that miss what they need

The ribbon buttons, drop-down rows, command search and the title bar's Solve read
Session::availability: greyed, with what is missing (and where to get it) in the tooltip.
Starting such an entry opens no form; the status bar names what is missing. ui.toolbar
reports each entry's `enabled` and `unavailable`, ui.inspect the Solve button's.

Fixes #220

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

docs/control-protocol.md

@@ -26,7 +26,7 @@ checked and clamped (drag steps, wheel lines, window size); `ui.render` refuses
 |---|---|---|
 | `engine.execute` | `command`: command id, `params`: JSON object | the command's result |
 | `engine.script` | `commands`: `[{command, params}, …]` | stops at the first error; `journal`: commands that changed the model so far |
-| `engine.commands` | — | every command: `id`, `label`, `params` (the parameter documentation) and `schema` (its parameters as JSON Schema) |
+| `engine.commands` | — | every command: `id`, `label`, `params` (the parameter documentation), `schema` (its parameters as JSON Schema) and `requires` (what the session must hold before it runs: `design`, `geometry`, `nodes`, `mesh`, `analysable`, `material`, `step`, `result`, …; `engine.execute` of a command missing one fails without doing anything, naming each missing piece and its toolbar entry) |
 
 Commands never open forms or file pickers when run this way. `fieldcraft-cli commands` prints the
 same registry. `file.sample {name}` opens a sample (`parts`, `cantilever`, `impact`) and
@@ -41,11 +41,11 @@ commands (files, undo, `model.info`) leave the ribbon where it is.
 
 | Method | Params | Notes |
 |---|---|---|
-| `ui.inspect` | — | view settings (`ui`), camera, viewport rect (points), renderer (`gpu`/`cpu`) and adapter, open command form, status bar, node and element counts, the results display (`results`: what the `results.*` commands reply: view, `stale`, contour range, modes), the probed node (`probe`), `busy` (the background job: a file opening, a solve, a mesher run from its form by a click on OK), `prompt` (`{"unsaved": path}`, `{"design": path}` (a design of another part opened over a meshed or defined model), `"recover"`, `"samples"`), `recent` (recent files), `selected` (the browser's selected definition), `nav` (the left-drag mode), `layout` (each region of the window drawn last frame as `[x, y, w, h]` in points: `app_bar`, `ribbon`, `workspace_switcher`, `document_tab`, `app_bar:<action>`, `browser`, `viewport`, `nav_bar`, `axis_view`, `selection_bar`, `context`, `command_line`, `timeline`, `toolbar_dropdown`, `palette`), version |
+| `ui.inspect` | — | view settings (`ui`), camera, viewport rect (points), renderer (`gpu`/`cpu`) and adapter, open command form, status bar, node and element counts, the results display (`results`: what the `results.*` commands reply: view, `stale`, contour range, modes), the probed node (`probe`), `solve` (the title bar's Solve: `enabled`, and `unavailable` as `ui.toolbar`'s entries give it, listing everything `solve.run` misses), `busy` (the background job: a file opening, a solve, a mesher run from its form by a click on OK), `prompt` (`{"unsaved": path}`, `{"design": path}` (a design of another part opened over a meshed or defined model), `"recover"`, `"samples"`), `recent` (recent files), `selected` (the browser's selected definition), `nav` (the left-drag mode), `layout` (each region of the window drawn last frame as `[x, y, w, h]` in points: `app_bar`, `ribbon`, `workspace_switcher`, `document_tab`, `app_bar:<action>`, `browser`, `viewport`, `nav_bar`, `axis_view`, `selection_bar`, `context`, `command_line`, `timeline`, `toolbar_dropdown`, `palette`), version |
 | `ui.set` | any view setting: `style` (`shaded_edges`, `shaded`, `wireframe`), `show_nodes`, `show_geometry`, `show_mesh`, `show_loads` (boundary condition and load symbols), `show_interactions` (reference points, couplings, rigid walls), `perspective`, `dark`, `show_tree` (the browser), `show_timeline`, `workspace` (`prepare`, `model`, `analysis`, `results`), `tab` (the toolbar tab: `geometry`, `mesh`, `properties`, `assembly`, `steps`, `loads`, `interactions`, `solve`, `results`, `tools`; a tab of another workspace brings that workspace), `palette` (the command search open), `menu` (the open menu: `file`, `help`, `workspace`, `view`, `more`, or a panel's drop-down as `<tab>/<panel>` such as `mesh/Tet`; `null` closes it) | an unknown setting or value is an error; returns the settings |
 | `ui.view` | `view`: `front`, `back`, `left`, `right`, `top`, `bottom`, `iso`, `home` (iso and fit), `fit` | returns the camera |
 | `ui.camera` | `yaw`, `pitch` (degrees), `distance?` (model units) | turns the camera to any orientation about its target: `yaw` about Z (0 looks along +Y, the front view; −45 is the iso view's, −90 the right view's), `pitch` the elevation above the XY plane; `distance` moves the eye (the zoom of an orthographic view). Angles must be finite and the distance above 0: the yaw is wrapped, the pitch clamped to ±90 and the distance to 0.001–10⁹; anything else is an error. A sequence of calls orbits smoothly. Returns the camera (`ui.inspect`'s, angles in radians) |
-| `ui.toolbar` | — | the ribbon: `workspace`, `workspaces`, `tab`, `tabs` (the workspace's), `menu` (the open menu), `panels` (each panel label's `menu` key and `rect`), and every catalogue entry (docs/features.md) with `id`, `label`, `tab`, `panel`, `command` (null while coming soon), `more` (its related commands), `coming_soon` (the milestone, or null), `fit` — `full` (a button), `collapsed` (its panel is one button), `more` (its panel is in the » menu) or `menu` (in its panel's drop-down) — and `rect` (screen points, last frame, on the shown tab; for `menu` entries the row while the drop-down is open) |
+| `ui.toolbar` | — | the ribbon: `workspace`, `workspaces`, `tab`, `tabs` (the workspace's), `menu` (the open menu), `panels` (each panel label's `menu` key and `rect`), and every catalogue entry (docs/features.md) with `id`, `label`, `tab`, `panel`, `command` (null while coming soon), `more` (its related commands), `coming_soon` (the milestone, or null), `enabled` (false while coming soon or while its command misses what it needs), `unavailable` (null, or what its command misses: `reason`, the tooltip's lines such as "Needs a material — Properties › Material", and `missing`, each with `requirement`, `what`, `provider` (the command that provides it) and `place` (its toolbar entry)), `fit` — `full` (a button), `collapsed` (its panel is one button), `more` (its panel is in the » menu) or `menu` (in its panel's drop-down) — and `rect` (screen points, last frame, on the shown tab; for `menu` entries the row while the drop-down is open) |
 | `ui.start` | `command`: a command or toolbar entry id | like a click on the toolbar, a drop-down row or a search result: a command without parameters runs, one with parameters opens its form in the context panel; an entry that is coming soon is an error naming its milestone. Never opens a native file picker: file entries (open, save as, write deck) open their command's form |
 | `ui.select` | `kind` (`set`, `material`, `section`, `step`, `boundary_condition`, `load`, `reference_point`, `coupling`, `fluid_domain`), `name`; nothing to deselect | selects a definition as a browser click does; its properties (with Edit and Delete) show in the context panel |
 | `ui.select` (viewport) | any of `filter`, `box_mode`, `mode`, `clear`, `at`, `box`, `geometry`, `nodes`, `elements`, `faces` (and no `kind`) | changes the viewport's selection (see [Selection](#selection)); returns `ui.selection`'s reply |