safe-bash: add `truncate` (set file size)

[kamilio]

Missing: truncate. No way to shrink/extend a file to an exact size.

Behavior: GNU truncate -s SIZE FILE incl. +/-/</>/% modifiers and -r REF/-o. Enforce the VFS quota/byte budget on growth (sparse-extend must still charge bytes, or refuse). VFS paths only.

From the safe-bash command gap analysis (2026-09-08). Sandbox constraints apply: bounded input/CPU/memory, VFS-only paths, cooperative cancellation, no host process/network beyond the existing curl authorizer.

[kamilio]

Implemented and verified on remote main at adc4d5b. Validation includes native differential/contract coverage, the maintained full unit route for the integrated utility changes, final build/lint/consumer checks, and freshly packed Node/Bun/browser-profile consumers. Truncate uses the explicitly selected Linux64 indexed-ext4 profile. This is scoped compatibility, not an all-platform or all-input equivalence claim. Release workflows 34395450680 (poe-code) and 34395450065 (scoped safe packages) are being monitored; publication is not yet claimed.