Quick info (Ctrl+G I, gi) shows what the name under the cursor is, in a box under the line #591

Context

Reading code, the only way to see what a name is (its type, signature, doc comment) is Go to definition and Back. This adds Quick info: one key asks the file's language server for hover at the caret and shows the answer in a box under (or above) the line. It reuses Go to definition's language-server plumbing and its status-bar messages. Pitch: #574.

Acceptance criteria

• Quick info is in the palette (F1), in the Go menu right after Go to definition with menu letter i, on Ctrl+G I, and on mnemonic gi. It only runs in an editor, and is greyed out like Go to definition while the file's language server isn't ready.
• In a C# file with the server ready, with the caret on a method call, Ctrl+G I opens a bordered box titled Quick info just below the caret's line, showing the method's signature highlighted with C# colours, then its doc comment as wrapped text.
• On a line near the bottom of the editor, the box opens above the line instead.
• The box is only as wide as its content needs, never wider than the editor, and at most 12 rows tall. When the answer is longer, ↑/↓/PgUp/PgDn scroll it.
• Its keys are buttons on its bottom border, Esc Close and Enter Go to definition, and clicking each does the same as the key. F1 while it's open lists them.
• Esc closes it; the caret hasn't moved.
• Enter closes it and jumps to the definition, exactly as gd would (Back returns).
• Any other key closes it and does what it would in the editor: typing x closes it and inserts x; → closes it and moves the caret.
• On whitespace or a keyword with no answer, no box opens and the status bar says Nothing to show here.
• While the server is still loading, or no server is configured for the language, the status bar says so the same way gd does, with gd's install hint.
• Pressing Ctrl+G I twice quickly on different names shows only the second answer.
• It works the same in another language with a server configured in Settings › Language Servers (e.g. Python or TypeScript), not just C#.
• Over SSH (or with the window resized narrow), the box still fits and draws without artefacts.

Tests

• The hover request is sent at the caret and the reply parsed for every shape the protocol allows: plain string, MarkedString, MarkedString[], and MarkupContent (markdown with fenced code, and plaintext). Fenced code becomes the highlighted signature; the rest becomes wrapped prose.
• Placement: below when there's room, above when there's more room above, width clamped to the editor, height to 12 rows.
• Scrolling keys move the content and are consumed; Esc, Enter and pass-through keys close it with the right side effect.
• An empty or null reply gives Nothing to show here; a stale reply (superseded request) is dropped.
• The command's enablement matches Go to definition's.
• The gi mnemonic and Ctrl+G I binding don't collide with existing ones.

Out of scope

• Opening on mouse hover.
• Signature help while typing a call's arguments.
• Merging answers from several servers.
• Links inside the doc comment, or rendering markdown beyond code vs prose.
• Peek definition (an embedded editor showing the declaration).