feat(builtins): add iconv command (encoding conversion)

[chaliy]

Summary

iconv converts text between character encodings. When processing files from unknown sources, encoding mismatches are a constant problem — mojibake, invalid UTF-8, Latin-1 legacy files.

Requested Flags

Flag	Description
-f encoding	Source encoding (from)
-t encoding	Target encoding (to)
-l	List available encodings
-c	Silently discard characters that cannot be converted
-o file	Output to file instead of stdout
//IGNORE	Suffix on -t to skip unconvertible chars
//TRANSLIT	Suffix on -t to transliterate unconvertible chars

Common Encodings to Support

Encoding	Description
UTF-8	Default, most common
ASCII	7-bit ASCII
ISO-8859-1 / LATIN1	Western European
UTF-16 / UTF-16LE / UTF-16BE	Windows Unicode
CP1252 / WINDOWS-1252	Windows Western European
ISO-8859-15	Latin-9 (Euro sign)

Use Cases

# Convert Latin-1 file to UTF-8
iconv -f ISO-8859-1 -t UTF-8 legacy.txt > modern.txt

# Pipe conversion
curl -s https://example.com/data.csv | iconv -f CP1252 -t UTF-8 | csvtool ...

# Strip non-ASCII characters
iconv -f UTF-8 -t ASCII//TRANSLIT input.txt

# List available encodings
iconv -l

Implementation Notes

• Rust's encoding_rs crate handles most common encodings
• Operates on VFS file contents (byte-level)
• Default target encoding should be UTF-8
• //IGNORE and //TRANSLIT suffixes on target encoding are important usability features

[chaliy]

Resolved in PR #585 or #588