fix(runtime-core): preserve u64 stat identity fields as bigint

[NathanFlurry]

• Keeps the protocol u64 stat fields (dev, rdev, ino, size, blocks, nlink) as bigint in LiveGuestFilesystemStat instead of throwing when a host filesystem reports values above Number.MAX_SAFE_INTEGER.
• Narrows to plain numbers only at the numeric VirtualStat boundaries (kernel proxy, rpc client, browser fs bridge), matching Node's default lossy fs.stat behavior.
• Brings the TS client stat shape in line with the Rust client, which already carries these fields as u64.
• Adds regression coverage for stat identity fields above the JS safe integer range.

Fixes the guest filesystem stat dev exceeds JavaScript safe integer range crash seen when walking host-mounted skills directories.

🤖 Generated with Claude Code

[railway-app]

🚅 Deployed to the agentos-pr-1652 environment in agentos

Service	Status	Web	Updated (UTC)
agentos	😴 Sleeping (View Logs)	Web	Jul 7, 2026 at 11:00 pm

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🚅 Environment agentos-pr-1652 in rivet-frontend has no services deployed.

[NathanFlurry]

Follow-up u64 narrowing audit fixes are pushed on stat-dev (aa35d976):

• Fixed WASI fd_pread/fd_pwrite positioned I/O offsets above 4 GiB by dropping the wasm32 >>> 0 truncation and adding the same finite/non-negative guard used by seek.
• Preserved exact bigint ino/size/nlink values through the stat-to-WASI filestat path while keeping the public VirtualStat number fields Node-compatible.
• Kept guest directory entry size as bigint in live response payloads, narrowing only at the public recursive-listing boundary.
• Converted stat timestamps with lossy Number(...) so far-future guest timestamps do not throw during host decode.

Validation artifacts were saved under ~/progress/agent-os/2026-07-07-stat-u64-bigint-fix/. Gates: pnpm build passed, pnpm check-types passed, runtime-browser unit Vitest passed, node scripts/verify-fixed-versions.mjs passed. Runtime-core Vitest only has the three noted pre-existing failures: protocol auth bytes, request-payloads maxDepth, and binary resolution.