safe-bash: add `iconv` (character-set conversion)

[kamilio]

Missing: iconv. No charset transcoding.

Behavior: iconv -f ENC -t ENC [-c] [//TRANSLIT] [FILE]. Support the encodings the platform TextDecoder/TextEncoder provide (UTF-8/16, latin1, ascii) and reject others with a clear unsupported error; -c drops invalid sequences. Bound input.

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]

Delivered directly to main at 18c36c5 after independent remote verification. Adds bounded VFS-only iconv with ASCII, Latin-1, UTF-8 and UTF-16 codecs, -c, and an explicitly documented C/POSIX/unset-locale //TRANSLIT profile. Root/scoped factories, default registration, limits and browser/type export routes are included. Validation: original glibc source review and finite native byte/status/diagnostic comparisons; 471 canonical source cases plus 10 focused public checks; separate 73 historical scratch executions against current source; full maintained workspace tests, lint, type and committed-archive gates; 22 fresh packed-consumer profiles / 43 commands, including Node/Bun, four type profiles and four zero-external browser bundles executed under Node. No universal locale/encoding or real-browser parity claim. Release publication is being monitored separately from this verified main delivery.