[Project516]

Pages can now cap the Java heap with cheerpjInit({ maxHeap: "256m" }). Before, the browser capped only a single array, so a program that kept allocating grew until the tab died. Under Node, -Xmx already caps the whole heap.

maxHeap takes bytes or a size with k, m or g, as -Xmx does: at least 2 MB, at most 1 GB. It sets what Runtime.maxMemory() reports, and CheerpJ has no such option.

How the browser measures the heap:

• The VM runs in a Worker. There, performance.memory and measureUserAgentSpecificMemory() are missing, so the VM counts live memory itself. It adds up arrays of 4 KB or more from allocation until a FinalizationRegistry callback reports them collected.
• The check works as it does under Node: a counter, a reading every step, and an error only when the cap is still passed.
• Browsers have no gc(), so a dropped array keeps counting until the engine collects it. Over the cap, the VM allocates a throwaway 256 MB ArrayBuffer, which makes Chromium collect, and pauses its threads a few milliseconds at a time. It throws OutOfMemoryError: Java heap space once finalizers have been quiet for 100 ms with the sum still over the cap, or after 1 s over. A smaller 64 MB push gave false errors.
• Without FinalizationRegistry, or without maxHeap, only the per-array cap applies.

Results in headless Chromium, filling a list with 256 KB arrays until the error, clearing it, churning garbage, and filling again, 8 rounds per cap:

maxHeap	false errors	held when stopped
32m	0 of 16	about 1.3x the cap
64m	0 of 8	about 1.12x
128m	0 of 8	about 1.09x
256m	0 of 8	about 1.06x

The error came about 0.1 s after the cap. Allocation-heavy code shows no cost from the counting: 400k 8 KB arrays plus 6M small objects ran in 1170 ms on master and 1180 ms here, within the 1069 to 1367 ms spread. The browser self-test's javac compile showed no difference beyond its run-to-run spread.

Tests:

• test/heapcheck.test.ts covers the counting and the lazy check.
• pnpm typecheck and pnpm test pass.
• docs/browser.md has a "Memory limit" section.

Noticed, not fixed:

• Objects and arrays under 4 KB do not count, so a program that only allocates those is not capped.
• Only Chromium was run. Firefox and Safari may collect later and stop a program further past the cap.

[coderabbitai]

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Files selected for processing: 11

• docs/browser.md
• docs/node.md
• src/browser/api.ts
• src/browser/protocol.ts
• src/browser/worker.ts
• src/heapsize.ts
• src/interp.ts
• src/jvm.ts
• src/node.ts
• src/runtime.ts
• test/heapcheck.test.ts

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[project516-review-bot]

This PR adds a maxHeap option to cheerpjInit() that caps the total Java heap in browsers, mirroring Node's -Xmx behavior. The browser worker tracks live memory by counting arrays ≥4 KB from allocation until FinalizationRegistry finalizers report collection. When the sum exceeds the cap, a 256 MB throwaway ArrayBuffer nudges Chromium to collect, and the VM pauses threads; OutOfMemoryError is thrown after 100 ms of finalizer quiet or 1 s total over the cap. Validation accepts bytes or -Xmx-style strings (k/m/g, 2 MB 1 GB). Documentation and tests are included.

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_{review-bot, model nvidia/nemotron-3-ultra-550b-a55b:free, verdict approve}