Fix `_burn` bigint blowup, ctx-mgr `_read_mhz`

Both `_burn()` impls (the `cpu-perf-check` script + the
`_measure_sustained_headroom()` probe in `tests.conftest`) grew `x`
~x**2 per iter, so the loop quickly went bigint alloc/mul-bound — a
noisy CPU load + needless memory across N procs. Mask each step to
64-bit for a steady, fixed-width ALU burn (still pegs every core,
which is all the freq probe needs).

Also, `_read_mhz()` opened the sysfs freq files without a ctx-mgr;
`with open(...)` so the FD closes deterministically (matches the
script's own `_read()`).

Review: PR #468 (Copilot)
https://github.com/goodboy/tractor/pull/468

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

scripts/cpu-perf-check

@@ -77,9 +77,13 @@ def _pkg_max_mhz() -> int:
 
 
 def _burn(stop: float) -> None:
+    # fixed-width (64-bit-masked) arithmetic keeps this a steady
+    # ALU load; an unmasked `x` grows ~x**2/iter into a huge bigint,
+    # degenerating into noisy alloc/mul-bound work (and needless
+    # memory) across N procs.
     x: int = 1
     while time.perf_counter() < stop:
-        x += x * x ^ 0x5
+        x = (x + (x * x ^ 0x5)) & 0xFFFF_FFFF_FFFF_FFFF
 
 
 def main(

tests/conftest.py

@@ -171,7 +171,8 @@ def _measure_sustained_headroom(
 
     def _read_mhz(path: str) -> int|None:
         try:
-            return int(open(path).read()) // 1000
+            with open(path) as f:
+                return int(f.read()) // 1000
         except OSError:
             return None
 
@@ -187,9 +188,13 @@ def _measure_sustained_headroom(
             return 1.
 
         def _burn(stop: float) -> None:
+            # fixed-width (64-bit-masked) arithmetic keeps this a
+            # steady ALU load; an unmasked `x` grows ~x**2/iter into
+            # a huge bigint, degenerating into noisy alloc/mul-bound
+            # work (and needless memory) across N procs.
             x: int = 1
             while time.perf_counter() < stop:
-                x += x * x ^ 0x5
+                x = (x + (x * x ^ 0x5)) & 0xFFFF_FFFF_FFFF_FFFF
 
         # explicit `fork` ctx so we're immune to whatever global
         # mp start-method tractor/the suite may have set (`spawn`