# Step: IOChan over files

Tag `v3-iochan-files`. Suite **130/130**; fixpoint **OK**
(`bootstrap/fixpoint.sh`, image **2,103,316 bytes**).

## What changed

`IOChan` now covers files as well as the standard streams, so a
program can treat a file and stdin/stdout uniformly through one
`ChanId`:

- `OpenRead(name)`, `OpenWrite(name)` → `ChanId` (a failed open yields
  a closed channel: `IsOpen` = FALSE).
- `Close(VAR cid)` — closes and clears the handle.
- `IsOpen(cid)`.
- `ReadString(cid, VAR s)` — reads one line (the newline is dropped)
  into the NUL-terminated string; `ReadInt(cid)` reads a signed
  decimal integer (skipping leading blanks).

The existing standard channels and operations (`StdIn/Out/ErrChan`,
`ReadChar`, `WriteChar`, `WriteString`, `WriteLn`, `WriteInt`) are
unchanged. Operations on a closed/failed channel stay safe no-ops
(`ReadChar` returns `0C`).

## Bridge additions

- `runtime/syslib/SysShim.def`/`.mod`: `freadint`, `freadline`.
- `runtime/syslib/shim.c`: `m2freadline` (line read into a descriptor,
  newline dropped, header set to the length). `m2freadint` already
  existed (used by `Files`).

## Test

`compiler/tests/iochanfile_prog.mod` (`IOChanFileProg`, exit 3):
writes `"line one\n-7\n"` through a channel to
`gen_ssa/_iochan_tmp.txt`, closes, reopens for reading, and checks the
line (`ReadString` → `"line one"`) and the integer (`ReadInt` → `-7`).
The harness removes the temporary file before and after.

## Notes

- `Close` takes `cid` by reference so it can set it to `NIL`; this
  mirrors the ISO-ish "release the channel" shape.
- `ReadString` truncates to the destination capacity.
- `Files` keeps its own direct shim binding for now; `IOChan` could
  become its base layer in a later step.

## Files

`stdlib/iochan.def`/`.mod`, `runtime/syslib/SysShim.def`/`.mod`,
`runtime/syslib/shim.c`, `compiler/tests/iochanfile_prog.mod`,
`compiler/run_tests.sh`, `docs/features.md`.