feat: add multi-file buffer foundation (v0.5 slice 1)

Rework the single Effect return into a drained effect queue
(Save{path,scope,contents} / Load / Publish / Pull) so one action can
ask the host for several ordered steps: opening a non-resident file
queues a Save of the outgoing dirty buffer then a Load of the target.

Keep the active buffer's fields inline on Editor and park inactive
buffers in a small LRU (<=3 resident = active + 2); switching back to a
resident buffer restores its caret/scroll/undo without touching the SD.
A dirty parked buffer is saved before eviction, so nothing leaves RAM
unsaved. `:e <path>` opens by prefix (/sd/repo -> Tracked, /sd/local ->
Local); `:sync` is refused in-core for a Local buffer.

Firmware drains the queue to empty each batch (a Load can cascade an
eviction Save) and persistence generalises the atomic save off the
hard-coded notes.md into load_path/save_path.

Also bump the side panel to FONT_9X15 and the `:` command line to
FONT_10X20 for legibility, word-wrapping the snackbar so a long notice
keeps its actionable tail.

CONTEXT.md

@@ -34,6 +34,25 @@ stay Local for their lifetime. Lives under `/sd/local/`.
 _Avoid_: draft, private, untracked, scratch (these all imply impermanence or
 promotability, which is not the model).
 
+### Editing model
+
+**Buffer**:
+A **File** loaded into memory for editing, with its own caret, scroll position,
+and undo history. Opening a file makes it the **active buffer** — the one the
+**Writing column** shows. Up to three buffers stay **resident** at once (the
+active one plus two parked in the background); switching back to a resident
+buffer restores its caret and undo without re-reading the card. A fourth open
+**evicts** the least-recently-used resident buffer — saved first if it has
+unsaved edits, so nothing is lost.
+_Avoid_: tab, window, document (a buffer is not a UI chrome element); "the file"
+when you mean the in-memory copy rather than the bytes on the card.
+
+**Open**:
+Bringing a **File** into the **active buffer**, via `Ctrl-P` (the file palette)
+or `:e`. Scope is read from where the file lives (`/sd/repo` → **Tracked**,
+`/sd/local` → **Local**), never chosen at open time.
+_Avoid_: load (implementation talk for the disk read behind an Open).
+
 ### User-facing actions
 
 **Save**:
@@ -68,7 +87,8 @@ _Avoid_: edit area, text area, main pane (superseded — they named the old
 full-width text region before the side panel carved out its right edge).
 
 **Side panel**:
-The right region (~160 px / ~25 cols, full height) holding all metadata:
+The right region (~160 px / ~17 cols at its FONT_9X15 metadata font, full
+height) holding all metadata:
 filename + dirty dot, word count, elapsed time, clock, Wi-Fi,
 keyboard-disconnect flag, publish state, and the mode indicator at its
 bottom-left. Sits entirely in the master half