feat(binds): unified bind transform for fzf >= 0.59

Add lua/fzf-lua/binds.lua (~740 lines) implementing the consolidated
bind transform system described in docs/specs/2026-03-27-unified-binds-design.md.

WHAT: All complex binds (lua functions, reload, exec_silent, builtin
actions, events) are routed through a single --bind=key1,key2,...:transform:CMD
entry point using shell.pipe_wrap_fn. Simple fzf-native actions (abort,
half-page-down, etc.) stay as direct --bind=key:action. Accept actions
use print(key)+accept so fzf can exit. Neovim-only keys (e.g. <C-Enter>)
use SIGWINCH bridge terminal keymaps.

WHY: Eliminates the need for separate convert_reload_actions and
convert_exec_silent_actions pipelines, and removes neovim terminal
keymaps for builtin actions (toggle-preview, toggle-help, etc.) when
fzf >= 0.59. All key dispatch goes through fzf's own bind system,
making binds work consistently regardless of focus.

KEY DESIGN DECISIONS:
- Gated to fzf >= 0.59 (can_unified check); legacy path unchanged
- Accept both old (keymap.fzf/keymap.builtin/actions) and new (binds)
  formats indefinitely, no deprecation
- 5 bind categories: Direct, Accept, Transform, SIGWINCH, Dropped
- Events use __evt__<name> prefix since $FZF_KEY is empty for events
- For builtin previewer without split, toggle-preview does NOT send
  fzf actions (fzf's preview pane is zero-width)
- { fn, actions.resume } backward compat converted to { fn, reload=true }
- Precedence: binds > actions > keymap.fzf > keymap.builtin

FILES CHANGED:
- lua/fzf-lua/binds.lua (new): normalize_binds(), build_transform_binds(),
  classify_bind(), _make_handler(), _execute_builtin(), _create_dispatch_handler(),
  can_unified()
- lua/fzf-lua/core.lua: import binds module, gate fzf_wrap to call
  binds.build_transform_binds when can_unified
- lua/fzf-lua/win.lua: unified path in setup_keybinds (SIGWINCH terminal
  keymaps, toggle-preview SIGWINCH handlers, Esc fix, early return);
  nil guard in on_SIGWINCH resize handler for window reuse safety

Author: ibhagwan