Pin per-isolate heap limit to V8 flags

Keep per-isolate heap limits in sync with the process-wide V8 --max-old-space-size and tune heap sizing for Next.js workloads. Changes include:

- Extract compute_old_space_cap_mb(...) and add SINGLE_WORKER_HARD_OLD_SPACE_CAP_MB to compute per-isolate old-space caps with dedicated single-worker behaviour.
- Adjust adaptive_per_isolate_heap_mb bands to larger values for realistic Next.js working sets.
- Wire a process-wide EFFECTIVE_HEAP_LIMIT (OnceLock<HeapLimitConfig>) seeded in apply_v8_flags and expose effective_heap_limit() to read it.
- Resolve per-isolate HeapLimitConfig from env or computed cap via resolve_effective_heap_limit and set it as the default used when booting V8 isolates.
- Apply the heap limit to new V8 BootContext instances (with_heap_limit calls) in dispatcher and engine pump so isolate.create_params().heap_limits(...) matches the V8 flag.
- Update and add tests to assert the new cap computations and resolution behaviour.

Motivation: prevent mismatches where the V8 process flag permits a larger old-space than each isolate's hard cap, which previously caused "Reached heap limit" aborts on real Next.js workloads; and provide more appropriate sizing for dedicated single-worker pods.

Author: sonquer

Cargo.lock

@@ -7,7 +7,7 @@ members = [
 ]
 
 [workspace.package]
-version = "0.1.27"
+version = "0.1.28"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 publish = false

Cargo.toml

@@ -275,7 +275,8 @@ async fn run_worker(
         .with_cjs(resolver)
         .with_cjs_root(ROOT_PARENT)
         .with_fs(crate::ops::FsHandle::real(vec![project_root]))
-        .with_process(ProcessConfig::builder(Arc::new(OsEnv)).build());
+        .with_process(ProcessConfig::builder(Arc::new(OsEnv)).build())
+        .with_heap_limit(crate::effective_heap_limit());
 
     let mut engine = match V8Engine::boot_with(&entrypoint, ctx).await {
         Ok(engine) => {