feat(session): persist and restore pane scrollback

Continue-where-you-left-off now brings the previous session's scrollback
back, not just the empty pane skeleton. Each pane writes its most recent
1500 lines to a bincode sidecar under
~/.config/kaku/session_content/<dir>/pane_<id>.bin; on restore the lines
reflow via Line::wrap when the column width changed and get prepended
into the freshly spawned shell's primary scrollback so the user's new
prompt stays at the bottom.

SNAPSHOT_VERSION bumps 3->4. A separate CONTENT_SCHEMA_VERSION lives
inside each sidecar plus a WEZTERM_SURFACE_FINGERPRINT constant lets the
cell payload evolve independently of the JSON envelope. All content
failures (missing sidecar, corrupt bincode, schema mismatch, alt-screen
pane) degrade silently to an empty scrollback so the structural restore
is never blocked.

Adds Pane::inject_scrollback (default no-op) backed on LocalPane by a
new TerminalState::prepend_primary_scrollback that always targets the
primary screen so a pane briefly on alt screen at save time still gets
its main-screen history when restored. After every envelope write,
gc_kept_content_dirs sweeps session_content/ down to just the
directories referenced by the two envelopes. Running processes (vim,
claude-code, ssh) cannot be revived; this is text only above a fresh
shell.

Author: tw93

Cargo.lock

@@ -114,7 +114,8 @@ wezterm-gui-subcommands.workspace = true
 wezterm-mux-server-impl.workspace = true
 wezterm-open-url.workspace = true
 # wezterm-ssh.workspace = true
-wezterm-term.workspace = true
+wezterm-term = { workspace = true, features = ["use_serde"] }
+bincode = "1"
 wezterm-toast-notification.workspace = true
 wgpu.workspace = true
 window-funcs.workspace = true