…r heap and arena (#24587)

Follow-up to #24565. Restructures the task so that adding a new heap or arena metric is a small, local change instead of edits scattered across measurement, diffing, rendering, and committed-value lookup:

- `HeapTracker` groups the global-allocator atomics into one static `HEAP` with `read()`/`reset()`; its doc comment describes how to add a heap counter.
- `Counters { heap: HeapCounters, arena: ArenaCounters }` replaces the flat `AllocatorStats`, with `record()` / `diff_since()` (the arena-chunk exclusion logic from #22621 is unchanged, just relocated).
- `StageStats::metrics()` is now the single declaration of what a snapshot contains: an ordered list of `Metric`s, each a label plus a `MetricKind`.
- `MetricKind` encodes the per-metric snapshot policy: `Count` (platform-exact), `ExactBytes` (exact, human-readable comment), `ApproxBytes` (tolerates cross-architecture layout noise by keeping the committed value within `APPROX_BYTES_TOLERANCE`, renamed from `ARENA_SIZE_TOLERANCE` since it is no longer specific to arena size).
- `write_snapshot` / `render_file_stats` are fully metric-driven; nothing downstream needs touching when a metric is added.

Adding e.g. a `sys deallocs` count is now: one `AtomicCounter` in `HeapTracker`, one field in `HeapCounters`, one `Metric::count(...)` entry. A new byte gauge (heap peak, arena capacity) is one gauge read plus one `Metric::approx_bytes(...)` entry.

Behavior is unchanged: the committed snapshots are byte-identical when regenerated with the refactored code.

🤖 Implemented with [Claude Code](https://claude.com/claude-code).

… peak growth (#24619)

Adds three heap-side metrics to the allocation snapshots, using the metric machinery from #24587:

- `sys deallocs` (exact count) — system deallocations per stage; read against `sys allocs` it shows whether a stage churns or retains heap.
- `sys alloc bytes` — cumulative bytes requested from the system allocator per stage (allocation sizes plus reallocation growth).
- `sys peak growth` — high-water mark of live heap during a stage relative to the live bytes at stage start, via live-bytes accounting in the `GlobalAlloc` wrapper. Captures transient heap a stage needs even when it frees it before finishing.

Arena chunk operations are excluded from all `sys` metrics at the source: `oxc_allocator` marks each chunk allocation/deallocation immediately before calling the system allocator (task-only `track_allocations` feature, compiled out everywhere else), and the task's tracking allocator consumes the marker and skips the call. This replaces the previous global chunk counter + per-stage subtraction: a subtraction can correct counts but not a high-water mark, and chunk sizes are quantized and platform-dependent (whether an arena needs one more chunk depends on byte totals that vary across architectures with type layout — the #22621 mechanism). The first CI run demonstrated this concretely: kitchen-sink's minifier peak differed deterministically by 720 kB between aarch64 and x86_64 because the mangler's temporary arena crossed a chunk-growth boundary only on x86_64; and antd's formatter "peak" was 762 MB of main-arena chunk-doubling history rather than formatter behavior (now 13.8 MB of actual heap use).

Cross-platform tolerances: the cumulative byte metrics still drift with the number of heap hashbrown tables allocated (their control-group width is architecture-dependent), so they use a relative tolerance (1%, 4 kB floor) via the committed-value comparison from #24565; `arena size` keeps its tight 1024-byte tolerance.

Sample values for checker.ts (2.9 MB source): parser needs only 2.4 kB of transient heap (fully arena-based), semantic peaks at 3.9 MB, the minifier is the pipeline's largest heap consumer at 10.2 MB, and the formatter requests 5.9 MB total while peaking at 3 MB (heavy alloc/free churn).

🤖 Implemented with [Claude Code](https://claude.com/claude-code).