fs_service: the transport is a ctx and three functions, not a *fuse.Fs

Step 2 of the 9P chain (docs/9p.typ 12.2, docs/registry.typ 9P-2).

`drain` and `step` never asked a `*fuse.Fs` for anything but `retry()`,
`next()` and `reply()`, so the concrete pointer was a coupling that bought
nothing and forbade a second answer. `Transport` names the three; `Fs.transport()`
is the first implementor and the thunks are the entire cost.

No behaviour change. The order contract -- retry() to null, then next() to null --
moves into `drain`'s doc comment, where it belongs: it is the caller's rule and
every implementor inherits it, rather than a fact about FUSE.

`start` and `wake` keep their `*fuse.Fs`: they are about a MOUNT, which is a
FUSE thing, and a 9P listener will bring its own.

Measured unchanged against zig build fs-bench -Doptimize=ReleaseFast: getattr 19 ns,
lookup 40, read body 4K/1M 25/25, read ctl 385, read index 633, readdir 38,
read event (empty) 22, all at zero allocations.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/gui/gui.zig b/src/gui/gui.zig
index c42e390b..da48d3fb 100644
--- a/src/gui/gui.zig
+++ b/src/gui/gui.zig
@@ -3735,7 +3735,7 @@ fn pollFrame(ctx: ?*anyopaque) void {
     // poll thread, so nothing else will wake us — re-arm the loop ourselves;
     // `Queue.push` is lossy for this variant, which is correct, because a queue
     // too full to take a wake is already holding one.
-    if (s.fs) |f| if (fs_service.drain(f, core).pending) s.queue.push(.fs_ready);
+    if (s.fs) |f| if (fs_service.drain(f.transport(), core).pending) s.queue.push(.fs_ready);
     const g = s.gui orelse return;
     // TaglineSize is pure renderer state: update the smaller face and its
     // visual band immediately, without changing the body metrics or grid.
@@ -3838,7 +3838,7 @@ fn gridPollFrame(ctx: ?*anyopaque) void {
     // something IS an event, so the pass renders: that is what lets a snapshot
     // `wait` for text a script wrote through the mount.
     if (s.fs) |f| {
-        if (fs_service.drain(f, s.core).count != 0) s.saw_event = true;
+        if (fs_service.drain(f.transport(), s.core).count != 0) s.saw_event = true;
     }
     pollCwds(s.core, s.ptys);
     // The harness polls stdin at the same 16 ms cadence as native SDL, so a