9proc core becomes a backend of cloud9.fs; engine gains optional features

9proc's own fid table, walk loop and dir-read engine are replaced by
cloud9.fs.Server; the tree (static, vars, providers) is served through the
engine's Req/Reply contract with node ids that keep the old qid scheme.
Providers may answer later by returning error.Again (parked in the engine,
retried each step, Tflush -> EINTR); no new files are exposed.
Engine (backward compatible, all opt-in via Backend.features / Options):
create, remove, wstat, reference accounting for backends that count
handles, a salted fid index, name_capacity 0 (names from getattr),
Reply.ename for backend-chosen error text, Attr.path/version/atime.
Engine-level error strings and the 217-byte msize floor now apply to 9proc;
tests updated accordingly.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

1 files changed, 7 insertions, 2 deletions

diff --git a/9proc/README.md b/9proc/README.md
index 0fd3ddb..15177d7 100644
--- a/9proc/README.md
+++ b/9proc/README.md
@@ -9,7 +9,12 @@ panics.
 
 The core (`core`, `vars`) is freestanding: no allocator, no OS, no threads,
 caller-owned static `Storage`, fixed-capacity tables sized at comptime. It
-compiles for `riscv32-freestanding-none`. `scratch` (an in-memory read/write
+compiles for `riscv32-freestanding-none`. The tree is a backend of
+`cloud9.fs.Server`, the file-server engine every 9P server built on cloud9
+shares: the engine owns fids, walks, permissions and directory cursors;
+the core answers its requests from static nodes, exposed variables and
+providers, and a provider may answer a read later (`error.Again` parks it
+until a later step). `scratch` (an in-memory read/write
 tree) takes an allocator; `linux` is the platform layer (listeners, a poll
 loop on one background thread, threads/stacks/registers, memory, breakpoints
 and panics via `std.debug`). [docs/LIBRARY.md](docs/LIBRARY.md) has the full
@@ -19,7 +24,7 @@ contract.
 9proc/
   build.zig          fragment imported by cloud9's root build.zig (steps below)
   src/root.zig       pub const core, vars, scratch, linux; Config, Server(cfg), Provider
-  src/core.zig       Tree/Server engine on cloud9.Server: fids, walks, dir reads, providers
+  src/core.zig       the tree (static nodes, vars, providers) as a backend of cloud9.fs.Server
   src/vars.zig       comptime value renderers (@typeInfo) for /vars
   src/scratch.zig    in-memory read/write tree provider (takes an Allocator)
   src/freestanding_check.zig   root for the riscv32-freestanding-none compile check