Implement feasible fzf.vim parity improvements

[vinitkumar]

Closes #6.

What changed

• adds ranked fuzzy matching and correct long-list viewport behavior to all extension pickers
• separates tracked Git files from Git-status files and restores fzf.vim-compatible :GFiles semantics
• parses NUL-delimited Git paths and rename records safely
• adds multi-select, quickfix, paste, and existing-window buffer actions
• implements fullscreen bang layouts and Git diff previews with file-content fallback
• documents the new commands, APIs, keymaps, compatibility aliases, and remaining parity gaps

Compatibility

• require('fff_plus').git_files() and :FFFPlusGFiles continue to open Git status files
• new code should prefer tracked_files() / :FFFPlusGitFiles and git_status() / :FFFPlusGitStatus
• legacy :GFiles now opens tracked files, matching fzf.vim
• all runtime code remains under lua/fff_plus/; upstream lua/fff/ ownership is unchanged

Commit structure

1. document the fzf.vim parity audit
2. add shared fuzzy matching and viewport foundations
3. separate tracked and status Git sources
4. add picker selection actions
5. add fullscreen layouts and Git diff previews
6. document public workflows
7. fix bottom-prompt ranking and expand regression coverage after review

Validation

• make format-check
• make lint
• make test
• git diff --check main...HEAD

Deferred follow-ups

New picker families—lines, history, marks, jumps, tags, commits, commands, help,
and completion—remain separate work after extracting a reusable extension picker
framework.