Module buffer_registry 

Unified buffer registry (DESIGN.md §5.9).

Every concrete buffer the App can route input through – a code Document today, a FileTreeBuffer tomorrow – lives in BufferRegistry keyed by BufferId. A single registry gives :bn / :bp / :ls / :bd a consistent surface across buffer kinds; multiple file trees coexist with multiple documents under the same shape.

Threading model (B’.1b)

BufferRegistry uses interior mutability via Arc<Mutex<BufferRegistryInner>>. Every method takes &self and locks briefly. The registry is Clone (cheap atomic bump) so the App’s BufferStore service impl can hold the same state as the App’s buffers field — modes call into the shared store from any thread; the App accesses the same data through its direct field.

Methods that previously returned Option<&BufferEntry> (and kind-specific equivalents like document(id)) are replaced with two flavours:

• Owned-return convenience methods for common patterns (document_handle, name_of, kind_of, flags_of, document_dirty, document_path, entry_summary).
• Callback methods (with_entry, with_document, etc.) for one-off access. The callback runs while the lock is held; callers MUST NOT re-enter the registry from inside (it would deadlock on the same Mutex).

Help buffers stay overlay-rendered for v1 (transient popup), so they’re not in the registry yet – moving them in is a follow-up that doesn’t require structural change.

Hot-path access: the active document’s actor handle, syntax state, and last-parsed-version live on [crate::app::App] directly so motion / dispatch code stays unchanged. Switching the active document snapshots those fields back into the matching registry entry and loads from the destination’s.

Structs

BufferEntry

One slot in the registry. The kind-specific data lives in BufferData; flags + id + name apply uniformly.

BufferRegistry

The App’s buffer registry. Methods take &self and lock internally; the registry is Clone so the App’s BufferStore service impl can hold a clone for cross-thread access.

DocumentEntry

Per-document registry payload. Each entry carries the actor handle plus per-document tree-sitter Syntax state, fold list, and any other “lives with this buffer until it closes” state.

Enums

BufferData

ListingKind

DL.4/DL.5: which listing kind a new listing Document is filed under. Both are DocumentEntry-backed; only the discriminator differs, which is exactly the PU.1a shape.