refactor: extract helpers, dedup patterns, add regression tests

Mechanical cleanup pass, behavior-preserving:
- remove dead code: reposition_bars (unused), sbar click infrastructure
  (zones never populated, handler iterated empty list)
- extract helpers in mode.lua: win_ok, buf_ok, is_dapui_pane_ft,
  for_each_dapui_pane (asymmetric filter preserved), safe_extmark,
  bar_set_lines, safe_set_height, if_still_active, invalidate_dead_bars,
  focus_source_win, unset_and_restore
- inline install_cs_listener inside define_highlights to dedup the three
  ColorScheme augroup+autocmd triples (italic, key, sep)
- data-drive sign_define in breakpoints.lua
- data-drive python interpreter candidate probing
- add tests/test_render_bars.lua pinning sbar/cbar output dimensions,
  zones, and overflow fallback behavior
- add tests/test_highlights_cs.lua verifying ColorScheme augroups and
  highlight group existence / re-apply on event
- add tests/test_helpers.lua unit-testing keyof and source_window
- expose M._test (direct function refs) so render/highlight internals are
  testable in headless mode

lua/turbo-debug/adapters/python.lua

@@ -2,31 +2,22 @@ local loader = require("turbo-debug.adapters.init")
 
 local M = {}
 
+-- Try in order: active virtualenv, conda, common local venv dirs, system.
+-- First executable match wins. Using a single ordered list keeps the probe
+-- sequence explicit (ipairs preserves numeric order).
 local function find_python()
-  -- check active virtualenv
-  local venv = vim.env.VIRTUAL_ENV
-  if venv then
-    local path = venv .. "/bin/python"
-    if vim.fn.executable(path) == 1 then return path end
-  end
-
-  -- check conda
-  local conda = vim.env.CONDA_PREFIX
-  if conda then
-    local path = conda .. "/bin/python"
-    if vim.fn.executable(path) == 1 then return path end
-  end
-
-  -- probe common venv directories relative to cwd
+  local candidates = {}
+  if vim.env.VIRTUAL_ENV then candidates[#candidates + 1] = vim.env.VIRTUAL_ENV .. "/bin/python" end
+  if vim.env.CONDA_PREFIX then candidates[#candidates + 1] = vim.env.CONDA_PREFIX .. "/bin/python" end
   local cwd = vim.fn.getcwd()
   for _, dir in ipairs({ ".venv", "venv", "env", ".env" }) do
-    local path = cwd .. "/" .. dir .. "/bin/python"
-    if vim.fn.executable(path) == 1 then return path end
+    candidates[#candidates + 1] = cwd .. "/" .. dir .. "/bin/python"
+  end
+  candidates[#candidates + 1] = "python3"
+  candidates[#candidates + 1] = "python"
+  for _, p in ipairs(candidates) do
+    if vim.fn.executable(p) == 1 then return p end
   end
-
-  -- fallback to system python
-  if vim.fn.executable("python3") == 1 then return "python3" end
-  if vim.fn.executable("python") == 1 then return "python" end
   return nil
 end