perf(web): disable icount by default (4.7x faster book open)

icount throttled TCG and starved the guest watchdog. Measured book open dropped from 146s to 31s. Off by default; ?icount restores it, ?noicount removed in favor of the new default. Harness now reports open-file timing.

ISSUES.md

@@ -12,9 +12,13 @@ Opening `/Test/example.txt` through QEMU `vvfat` could corrupt coroutine/guest s
 
 Opening a book panics **only in WASM** (`Invalid read ... reason: rejected` → Load access fault) inside `uzlib_uncompress` during the reader's font `prewarmCache` **if** compiled guest stores are enabled. Native QEMU from the same submodule renders fine.
 
-Root cause (confirmed by full diff against pristine upstream): `wasm_tci_only_tb = true` in `tcg_out_qemu_st` is **upstream qemu-wasm's own design**. Upstream forces every store-containing TB to the TCI interpreter because its compiled restartable-block store path is not rewind-safe (a yielded softmmu helper replays the block; safe for loads, corrupting for stores). This is the dominant cost of book open (font inflate is store-heavy, so the hot loop never JIT-promotes).
+Root cause (confirmed by full diff against pristine upstream): `wasm_tci_only_tb = true` in `tcg_out_qemu_st` is **upstream qemu-wasm's own design**. Upstream forces every store-containing TB to the TCI interpreter because its compiled restartable-block store path is not rewind-safe (a yielded softmmu helper replays the block; safe for loads, corrupting for stores). Font inflate is store-heavy, so that hot loop never JIT-promotes — a real but secondary perf ceiling.
 
-The real fix is to make compiled stores rewind-safe and drop the blanket TCI — a substantial TCG-backend task, ~15 min per browser iteration, no native repro. Full analysis: `docs/wasm-inflate-panic.md`.
+Remaining fix is to make compiled stores rewind-safe and drop the blanket TCI — a substantial TCG-backend task, ~15 min per browser iteration, no native repro. Full analysis: `docs/wasm-inflate-panic.md`.
+
+## Resolved: `-icount` throttled book open ~4.7x
+
+The browser ran with `-icount shift=auto,align=off,sleep=off`, which serialized TCG and starved the guest watchdog (the earlier `wdt timeout` panics). Measured book open: 146s with icount, 31s without. icount is now off by default in `web/app.js`; `?icount` restores it. Guest timekeeping tracks host wall-clock, which is fine here because running flat-out feeds the watchdog faster, not slower.
 
 ## Resolved: reader grayscale rendered inverted and muddled
 

tests/browser_raw_sd.py

@@ -163,7 +163,9 @@ try:
     press(confirm, 'opening Browse Files')
     press(confirm, 'opening Test directory')
     press(confirm, 'highlighting example.txt')
+    open_start = time.monotonic()
     press(confirm, 'opening example.txt')
+    open_seconds = time.monotonic() - open_start
 
     down = driver.find_element(By.CSS_SELECTOR, '[data-button="down"]')
     up = driver.find_element(By.CSS_SELECTOR, '[data-button="up"]')
@@ -187,7 +189,8 @@ try:
     print(
         f'browser {MODE} test passed at frame generation '
         f'{final_state["generation"]}, '
-        f'wasm heap {final_state["wasmHeap"] / 1024 / 1024:.1f} MiB'
+        f'wasm heap {final_state["wasmHeap"] / 1024 / 1024:.1f} MiB, '
+        f'open example.txt took {open_seconds:.1f}s'
     )
 except Exception:
     state = save_artifacts()

web/app.js

@@ -462,14 +462,17 @@ async function boot(file, variant, setStatus) {
   logLine(sdMessage);
   const { default: createQemu } = await import(qemuUrl);
 
-  const qemuLogFlags = new URL(location.href).searchParams.has('trace')
+  const params = new URL(location.href).searchParams;
+  const qemuLogFlags = params.has('trace')
     ? 'guest_errors,unimp,in_asm'
     : 'guest_errors,unimp';
+  // icount throttles TCG ~4.7x here and starves the guest watchdog; off by default. ?icount restores it.
+  const icount = params.has('icount') ? ['-icount', 'shift=auto,align=off,sleep=off'] : [];
   const options = {
     arguments: [
       '-machine', `xteink,variant=${variant}`,
       '-accel', 'tcg,tb-size=16',
-      '-icount', 'shift=auto,align=off,sleep=off',
+      ...icount,
       '-L', '/bios',
       '-display', 'none',
       '-serial', 'stdio',