docs(load-testing): correct the ledger after adversarial review

[NathanFlurry]

Records F-8/F-9/F-10 and corrects two claims this file made:

• F-7's premise that the offending write "was already correctly rejected
  with a typed ENOSPC at the syscall" is false. F-5 is per-file rather than
  per-VM, misses writeFileSync/copyFileSync entirely, and natively spawned
  processes write the shadow dir with unmediated host syscalls.
• The filesystem-bytes probe passing verified only the byte-limit-on-
  regular-file case and must not be cited as closing the class.

[NathanFlurry]

Stack for rivet-dev/agentos

• #1815 docs: document sidecar stderr forwarding and host-backed fil …
• #1814 fix(native-sidecar): stop stranding child process.fd_read on …
• #1813 fix(native-sidecar): close maxFilesystemBytes gaps on host-b …
• #1812 docs(load-testing): correct the ledger after adversarial rev … 👈
• #1811 fix(native-sidecar): close remaining guest-reachable fatal p …
• #1810 fix(runtime-core): bound retained sidecar stderr and always …
• #1809 fix(native-sidecar): always answer sync-RPC waiters on handl …
• #1804 fix(native-sidecar): tolerate guest filesystem-quota errors …
• #1806 fix(native-sidecar): stop routing mapped host fds into the k …
• #1807 fix(native-sidecar): enforce maxFilesystemBytes on mapped ho …
• #1802 fix(native-sidecar): keep guest-triggered errors from termin …
• #1801 feat(runtime-core): forward live sidecar stderr via AGENTOS_ …
• #1805 fix(runtime-core): handle stdin error events in StdioFrameTr …
• #1803 test(load): add bounded Docker load-testing harness and issu …
• main

Get stack: forklift get 1812
Push local edits: forklift submit
Merge when ready: forklift merge 1812

_{change xwxulsum}