feat: add zero-width character plugin

[Lalatenduswain]

Summary

Adds a zero-width obfuscation plugin that interleaves invisible zero-width Unicode characters between the visible characters of the text.

To a human reader (and in most UIs) the text looks unchanged, but the underlying byte sequence no longer contains any contiguous ASCII substring — defeating naive keyword/regex filters, substring denylists, and byte-level matching. It is the interspersing counterpart to the recently-added homoglyph plugin:

• ascii_smuggler replaces text with invisible Unicode tag characters (visible content disappears).
• homoglyph substitutes Latin glyphs with look-alike confusables (every visible code point changes).
• zero_width keeps the original visible glyphs intact and injects invisible separators between them.

Options

• char — which zero-width character to insert: zwsp (U+200B, default), zwnj (U+200C), zwj (U+200D), wj (U+2060), zwnbsp/bom (U+FEFF).
• ratio — fraction of inter-character gaps to fill, 0.0–1.0 (default 1.0, deterministic).
• seed — optional integer seed for reproducible partial output when ratio < 1.0.

Changes

• spikee/plugins/zero_width.py — new plugin (subclasses BasicPlugin, so exclude_patterns handling is inherited; tagged Encoding, Obfuscation). Style mirrors homoglyph.py.
• tests/functional/test_spikee_list.py — assert zero_width is discoverable via spikee list plugins.
• docs/02_builtin.md — add zero_width to the built-in plugins reference table.

Usage

spikee generate --plugins zero_width
spikee generate --plugins zero_width --plugin-options "zero_width:char=zwnj"
spikee generate --plugins zero_width --plugin-options "zero_width:ratio=0.5,seed=42"

Testing

• Verified via the module loader: visible text is preserved (strips back to the original), no contiguous ASCII substring remains, char/ratio/seed behave, and exclude_patterns tokens are preserved verbatim.
• spikee list plugins shows zero_width under Encoding, Obfuscation.
• All functional tests pass locally on the develop base (116 passed: test_spikee_list, test_module_loading, full test_spikee_generate).