feat(playground): tracer v2 records traces in the shared snapshot

[mnaimfaizy]

PR 5 of the ADR 0004 build plan. A new module at src/features/playground/tracer/, added beside the old tracer and not wired into the app; the swap comes with the stack-and-heap lens.

What it does

• Rewrites the learner's JavaScript (acorn + astring, no new dependency), runs it on the browser's own engine and returns a trace: steps that each hold a shared snapshot (src/shared/snapshot).
• Every object, array and closure keeps one identity for the whole run, so two variables holding one array give two slots with the same refId.
• Every frame on the stack carries its own slots.
• Functions are traced wherever they are written: declarations, expressions, arrows, object and class methods, callbacks run by built-ins, async functions, generators.
• The step limit (2,000 by default) stops the run and keeps the partial trace, including while(true){}, a loop that catches everything, and a finally that returns.
• A step's focus is derived from what the step created or changed.
• Limits on slots per object, string length and heap objects per snapshot.

Evidence

• story.test.ts: tracing the Stack ↔ Heap story's code gives the references its beats assert for the 9 beats a run can observe. The 10th is the garbage-collection sweep, which a trace never shows.
• examples.test.ts: the Playground's JavaScript examples and default template trace without error and print what the untraced code prints.
• The runtime is one self-contained typed function delivered as createTracer.toString(); tests run the serialised source standalone, and again after esbuild minification.
• Type check, lint, 480 tests in 43 files and the build pass.

Known limits

• A step is recorded after its statement ran, so its line and focus describe the same statement.
• Top-level functions and classes show as values (ƒ, class); only closures become heap objects.
• yield* resumption can miss the outer frame; the scope a class method closes over is not drawn; let/const declared directly in a switch case are not shown.
• A function holding a var and a function declaration of the same name is refused; top-level await is a syntax error; code that declares its own __t fails with a confusing message.
• Timers and promises stay native here; the queues come with the event loop PR.
• examples.test.ts looks for the examples file under both its old and new name while the rename in the Playground basics PR is in flight; tidy once both are merged.

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