feat: support multi-character input delimiters

[vmvarela]

Summary

Closes #86.

Extends -d/--delimiter to accept strings of 1–8 bytes instead of a single character. Common real-world separators like ||, ;;, or two spaces now work without preprocessing.

Changes

src/csv.zig — streaming parser

• delimiter: u8 → delimiter: []const u8; added partial_delim: usize = 0 to track in-progress multi-byte matches
• State machine updated in .field_start, .unquoted, and .quote_saw: when delimiter[0] is seen, bytes are tentatively consumed; on full match the field is flushed, on mismatch the prefix is emitted as literal field content and the current byte is re-evaluated
• EOF handler flushes any pending partial-match bytes before closing the record
• partial_delim is reset at the top of nextRecord to guard against stale state after a non-fatal I/O error
• +11 unit tests covering: 2-char, 3-char, partial false-positive, quoted field containing delimiter, empty first/last field, only-delimiter input, EOF without newline, partial at EOF, greedy left-to-right behavior

src/main.zig — CLI and output

• parseDelimiter: now returns []const u8; accepts 1–8 bytes, rejects empty and len > 8
• All delimiter: u8 fields updated across ParsedArgs, ColumnsArgs, ValidateArgs, SampleArgs
• writeField/printRow/printHeaderRow: delimiter type updated to []const u8; quoting detection uses std.mem.indexOf instead of char-by-char comparison

build.zig

• Integration tests 95–98: || delimiter, ;;; delimiter, empty delimiter error (exit 1), delimiter > 8 chars error (exit 1)

Docs

• README.md and docs/sql-pipe.1.scd: updated to reflect 1–8 character range, added multi-char examples

Behavior notes

• Single-character delimiters are fully backward compatible (fast path: delimiter.len == 1 avoids partial-match overhead)
• Matching is greedy left-to-right; overlapping patterns (e.g. aa in aaa) follow this documented strategy
• --tsv remains a shorthand for --delimiter $'\t' (single char, unchanged)
• --sample output uses the same delimiter as input (consistent with existing single-char behavior)

Testing

zig build test   # all integration + unit tests
ziglint src build.zig