Implement `<ProgressBar>` tag (egui::ProgressBar)

[ZhukMax]

Add a declarative <ProgressBar> for displaying task progress. The widget takes a normalized value in [0.0 … 1.0], optional text, and common UX options like percentage display, width hint, disabled state, and tooltip.

• Covers a ubiquitous “in-progress” UI pattern for downloads, imports, long-running jobs.
• Keeps templates concise and consistent with EFx’s attribute model.
• Plays well with live updates in native and wasm32-unknown-unknown builds.

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Scope

• Tag: <ProgressBar>
• Required: value={number_expr} (float in 0..=1; clamped at render if out of range).
• Optional attributes:
  text="...", showPercentage=bool, desiredWidth=f32, enabled=bool, tooltip="...", id="...".
• Events: none (display-only), but the Response should allow hover tooltips.
• Children: none (text is via text="...").

Non-goals (here): indeterminate/animated barber-pole, custom colors/skins.

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Proposed API (sketch)

Template

efx!(ui, r#"
<ProgressBar value={job.progress} text="Importing…" showPercentage="true" desiredWidth="240"/>
"#);

Conceptual mapping to egui

let v = (value as f32).clamp(0.0, 1.0);
let mut pb = egui::ProgressBar::new(v);
if let Some(t) = text           { pb = pb.text(t); }
if show_percentage              { pb = pb.show_percentage(); }
if let Some(w) = desired_width  { pb = pb.desired_width(w); }

let resp = if enabled { ui.add(pb) } else { ui.add_enabled(false, pb) };
if let Some(tip) = tooltip { resp.on_hover_text(tip); }

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Tasks

• AST & parsing

  • Add <ProgressBar> node with attributes: value, text, showPercentage, desiredWidth, enabled, tooltip, id.
  • Reject unexpected children; error if both text attribute and children are supplied (children not allowed).

• Codegen

  • Read value={expr} (float-like), clamp to [0.0, 1.0] at render time.
  • Map text, showPercentage, desiredWidth, enabled, tooltip, id to the egui builder and Response.

• Diagnostics

  • Missing value → clear compile error with expected signature.
  • Non-numeric value → type error with hint.
  • Children present → error: <ProgressBar> doesn’t take children; use text="...".
  • Unknown attributes → list allowed attributes.

• Examples & docs

  • examples/progress_bar.rs: simulated task updating progress over time; with/without percentage.
  • Add snippet to docs/cookbook.md and entry to docs/tags.md.

• Tests

  • trybuild UI tests: missing value, wrong type, children supplied, unknown attrs.
  • Ensure example builds for wasm32-unknown-unknown (no runtime launch).