Hidden-class transition visualizer built on --trace-maps

[pavlof01]

Context

--trace-maps and --trace-maps-details are already in the shared flag catalog (packages/engine-runtime/src/flags.ts, category object-shapes), so the plumbing is done — a user can turn them on today and gets a wall of raw text.

Hidden classes are the most visual topic in V8 internals and nobody shows them interactively.

Proposal

• Parse --trace-maps output into a transition graph: map addresses as nodes, property additions / element-kind changes / prototype transitions as edges.
• Render the graph on its own page, replayed step by step in sync with the source, showing how {} grows into its final shape and where two objects diverge into different shapes.
• Highlight the moment two objects that "look the same" end up on different maps — the payoff case.

Acceptance criteria

• A snippet building the same object in two different property orders shows two divergent shape chains.
• Parsing is unit-tested against recorded --trace-maps output; no engine spawn in tests.
• No infrastructure change: the flags are already allowlisted.