refactor(peek): replace adapter registry with push-model peek sources

[WilliamHsieh]

Summary

Replaces the peek subsystem's name-keyed adapter registry with a "push" model: a peek source is just a function that builds OverlookPopupOptions and hands them to require("overlook.window").open_popup(opts). No dispatcher, no contract object, no .async flag, no string-keyed registry, no name lookup.

The old design dispatched through a table keyed by adapter name (default_adapters[name] / Config.adapters[name]), resolved at call time. That was stringly-typed, invisible to lua-ls, and existed mainly to support custom-adapters-by-name — an extension surface that isn't a goal pre-1.0. The built-in dispatch never needed it: the names are fixed at author time and the module is the handle.

Pre-1.0, internal cleanup, adapter contract free to break.

What changed

• Deleted lua/overlook/peek.lua (the metatable __index dispatcher).
• adapter/ → peek/: cursor, marks, definition are now plain functions. require("overlook.peek.cursor")() reads as the action and mirrors the peek_cursor API.
• The sync/async split disappears. Sync sources call open_popup directly; async sources (LSP) call it later from their own callback. There's no .async flag or two-shape contract anymore.
• window.lua: added module-level M.open_popup(opts) = M.current():open_popup(opts) — the single seam a source uses, so a source never has to know the Window/Stack/Popup layering. It lives where popups already live rather than in a separate peek layer.
• api.lua: peek_* wrappers call the source modules directly; dropped the Peek require.
• config.lua: removed the adapters option (and the your_custom_adapter stub that deep-merged into every user's config as a callable no-op). A custom source is now "write a function that calls Window.open_popup and bind it to a key" — no registration.
• types.lua: dropped the OverlookAdapter contract; a peek source is documented as a plain function (OverlookPeekSource).

Bug fixes folded in

• peek_mark no longer fires the stray pre-prompt marks() call that produced a double error on every invocation.
• definition guards empty/nil LSP results and missing user_data (was an attempt to index a nil value crash on the common no-definition case).
• cursor drops the dead file_path field (set but never read, not in OverlookPopupOptions).
• A source with nothing to peek notifies and simply does not open; no generic "returned nil options" error stacked on top.

Test plan

• Per-source specs (cursor_spec, marks_spec, definition_spec) assert each source calls Window.open_popup with the right options, or notifies and opens nothing.
• api_spec covers peek_mark's prompt flow (no pre-prompt fire; one fire on a valid char; invalid/cancelled handling).
• The dispatcher spec is removed (the dispatcher is gone).
• 99 tests pass; stylua clean.

Breaking changes

• Config.adapters is removed. Any custom adapter registered via config no longer resolves — register a custom source by binding a function that calls require("overlook.window").open_popup(opts) to a key instead. Acceptable pre-1.0.

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