feat(toolbar): add selection-triggered inline floating toolbar (closes #327)

[fengguoheng]

Closes #327.

Summary

Adds an opt-in inline floating toolbar to plugin-toolbar that appears when the user selects text inside the editor - the same affordance Notion / Google Docs / Medium offer for inline formatting.

Mirrors the proven lifecycle of createSlashMenuUI (selection-triggered, viewport-flipping, IME-aware, dismiss-on-outside-click) and reuses the existing ToolbarButton / ToolbarGroup types and toolbar-commands actions so hosts can compose their own button sets with zero new dependencies.

What is included

• createInlineToolbarUI(editor, options) in packages/plugin-toolbar/src/inline-toolbar.ts returning { element, show, hide, destroy }
• Shows when the editor reports a non-empty selection (getSelectedText() is not empty); hides when the selection collapses
• Centers above the selection via editor.getCoordsAtPos(), flips below when the viewport has no room above, clamps inside the right edge
• Default buttons are inline-only - bold, italic, strikethrough, inline code, insert link - so the inline toolbar never offers block-level commands that do not make sense on a selection
• Hosts may pass custom groups, a custom mount container, a classPrefix for styling hooks, and a vertical offset
• Full lifecycle parity with createSlashMenuUI: destroy() detaches all DOM listeners and the element; safe to call multiple times. IME composition suppresses show; compositionend re-evaluates. Escape and outside-mousedown dismiss; re-selecting resets the latch. Editor blur dismisses. Window resize and scroll re-position
• Exports createInlineToolbarUI, InlineToolbarUI, InlineToolbarUIOptions from the package entry; re-exports ToolbarButton / ToolbarGroup for hosts building custom groups
• OpenSpec proposal at openspec/changes/add-inline-toolbar/ (proposal.md, tasks.md, specs/plugins/spec.md) with 9 ADDED Requirements and 17 scenarios

Why this design

• No new package, no new dependencies. The inline toolbar lives next to the existing top toolbar in plugin-toolbar and reuses its action catalogue
• No core API additions. Uses only the existing selectionChange event, getCoordsAtPos, getSelectedText, getSelection, isComposing, focus, blur, on / off surface
• Mirrors createSlashMenuUI. The slash menu lifecycle (dismiss latch, IME guard, viewport flip, capture-phase Escape, capture-phase outside-mousedown, resize reposition, idempotent destroy) is the codebase reference pattern for floating editor overlays - reusing it keeps behavior consistent and reviewable

Test plan

• packages/plugin-toolbar/test/inline-toolbar.test.ts - 22 tests covering lifecycle (mount/show/hide/destroy/idempotent destroy/custom container), default groups (inline-only buttons + bold/italic wrapping), custom groups (render order, action invocation, mousedown focus retention, custom class prefix), dismissal (Escape latch reset, outside mousedown, mousedown inside), composition (suppress on start, re-eval on end), and viewport changes (resize/scroll)
• pnpm --filter @floatboat/nexus-plugin-toolbar exec tsc --noEmit passes clean
• pnpm test packages/plugin-toolbar - 66/66 tests pass (22 new + 44 existing, no regressions)
• Manual smoke test in the electron demo

[CLAassistant]

All committers have signed the CLA.