Rebuild the Virtual Environment Runner card

[dovvnloading]

Problem

The Virtual Environment Runner card clipped its own content at every level.

• The body is a max-height flex column, and a flex container under height pressure shrinks its children below their content size unless told not to. Every textarea and section was being compressed until it cut its own text mid-glyph: a two-row requirements box showing 1.6 rows, the prompt cut through its descenders, the terminal showing half a line.
• Requirements, prompt, code and terminal each carried their own scroll region inside the scrolling body - up to five nested scrollbars on one 420px card.
• The requirements manifest sat first, permanently open, above the prompt.
• A permanent "No output yet." terminal rendered on every fresh node.
• "Code" and "PYTHON" stacked two all-caps headers over the same pane.
• Run was a neutral chip identical to Cancel, and kept reading "Run", merely disabled, while a run was in flight.

Change

• flex-shrink: 0 on every body child - the mid-glyph clipping's actual cause.
• The prompt leads. Both inputs auto-size to their content (field-sizing: content, capped, no resize handles); an input never scrolls what was just typed.
• Requirements fold into a native <details> whose summary carries the package count. The blur/Enter commit contract is unchanged.
• Only the two outputs scroll themselves (code 185px, terminal 150px), and the card budget (560, body 520) is spent so the common full state - code plus terminal - fits with no outer scrollbar. An outer scroll on this card is what cut through the middle of whichever pane straddled its edge.
• The terminal renders only when a run is in flight or output exists.
• The duplicate "Code" label is gone; the fence's own PYTHON header names the pane once. Run is the card's one filled primary and reads "Running…" while busy; Cancel is quiet with a destructive hover.
• Unchanged: all intents, the live-stream subscription, the approval panel wiring, and the machine-output posture of the terminal (plain preformatted text, never the markdown pipeline).

Test plan

• CodeSandboxNodeView.test.tsx: 61 pass. Two assertions updated for the deliberate changes: the busy-state label ("Running…" disabled, rather than a disabled button still reading "Run"), and the removed duplicate header (asserted as absent, with the pane's presence checked by class).
• npm run check (schema drift, typecheck, lint, vitest, build, bundle size) against a clean checkout of this branch: 2135 pass.
• Manual against the built bundle with the incident fixture: the prompt, folded dependencies row with count, single-header code pane and terminal all render unclipped at 147% zoom; the full card (code + terminal populated) shows no outer scrollbar; opening the dependencies disclosure auto-sizes the manifest with no inner scroll.