fix: deferred dep loading + idempotent setup

Two related bugs surfaced when the user tried adding `setup({...})` to
their init.lua alongside our auto-setup:

1. **`inmemory.bps = nil` crash.** When a user's `init.lua` does
   `vim.pack.add(..., turbo-debug)` and then calls `setup({...})`
   synchronously right after, the nested `vim.pack.add` calls inside
   our own `plugin/turbo-debug.lua` (for dap/dapui/persistent-
   breakpoints) are DEFERRED by nvim's pack module — so the user's
   follow-up `require('persistent-breakpoints')` throws "module not
   found". On the previous `initialized = true` guard, the error set
   the flag but left setup half-complete; any later call short-
   circuited and `inmemory.bps` stayed `nil`, exploding the first
   breakpoint toggle (`api.lua:15: attempt to index field 'bps'`).

2. **Colorscheme override ignored.** Same root cause: user's
   `setup({ colorscheme = "nightfox" })` errored before reaching
   `config.merge`, so `config.opts.colorscheme` stayed `nil` and
   `M.enter` never switched themes.

Three changes that together fix both:

- **`init.lua` `setup()` self-defers** when `require('persistent-
  breakpoints')` fails, `vim.schedule`s itself once deps can load.
- **`plugin/turbo-debug.lua` auto-setup is now `vim.schedule`d** and
  skips entirely if the user's explicit `setup({...})` already ran
  (tracked via `M._setup_called`) — no more stomping user opts with
  a second `config.merge(nil)`.
- **`breakpoints.setup` is now fully idempotent**: dropped the
  `initialized` flag, since every call inside it
  (`persistent-breakpoints.setup`, `sign_define`, `nvim_set_hl`,
  `vim.keymap.set`) is safe to re-run.

plugin/turbo-debug.lua

@@ -16,5 +16,18 @@ pcall(function()
   require("which-key").add({ { "<leader>d", group = "debug" } })
 end)
 
--- setup immediately (idempotent — user can call setup() again with overrides)
-require("turbo-debug").setup()
+-- Deferred auto-setup. Must be deferred (not called synchronously) because
+-- the vim.pack.add calls above might not have finished registering the
+-- dep modules in runtimepath by the time this script's top-level runs —
+-- a synchronous `require('persistent-breakpoints.api')` inside setup would
+-- crash with "module not found". vim.schedule guarantees we run AFTER the
+-- current event loop tick, at which point pack has finalized the paths.
+--
+-- If the user calls setup({...}) themselves (typically from init.lua after
+-- vim.pack.add), `M._setup_called` flips true and this deferred call
+-- becomes a no-op — otherwise we'd overwrite their user opts with defaults.
+vim.schedule(function()
+  local td = require("turbo-debug")
+  if td._setup_called then return end
+  pcall(td.setup)
+end)