# V3 step 5.2 — HIGH/LEN + Strings/TextIO stdlib (done 2026-09-22)

Second stdlib landing. Suite 90/90 (47 run — incl. 4 trap-aborts,
4 sessions, 2 session-output, 1 unit — + 43 reject).
LL(1)-clean, zero gm2 warnings.

## HIGH / LEN builtins

Pulled forward from step 6 because string libraries need the
descriptor length at runtime:
- Fixed array: `HIGH`/`LEN` fold to immediates from the static
  bounds (`ArrayHi`/`ArrayLen`).
- Open array (formal): read the header — `LEN` = `loadl` count,
  `HIGH` = `loadl count - 1` (new `QbeGen.LoadCount`).
- Non-array → 217. Works on nested designators (`HIGH(m[i])`),
  since `Design` yields the inner descriptor address.
- `t_highlen` → 18 (`HIGH(a)+LEN(a)+F(a)` with a 5-element array).

## Standard library (`stdlib/`, classic form)

- `sysio.def/.mod` — `Write`, `WriteLn`, `WriteInt`, `WriteChar`
  over the shim externals (moved out of `tests/` into `stdlib/`).
- `strings.def/.mod` — `Length`, `Assign`, `Concat`, `Compare`,
  `Copy`. The descriptor header holds the **logical length**:
  `Assign`/`Concat` call the new shim `m2setlen` so `Length`
  reports content, not capacity (otherwise a fixed `ARRAY [0..15]`
  would always report 16). Writes are capacity-bounded (truncate,
  never overrun).
- `textio.def/.mod` — ISO-style `WriteString`/`WriteLn`/`WriteInt`/
  `WriteCard`/`WriteChar`, binding the shim directly (avoids the
  flatten-on-import name shadowing that would make a wrapper named
  the same as its SysIO target recurse).

## Shim additions (`runtime/syslib/shim.c`)

`m2setlen(desc, n)` (set logical length), `m2writechar(c)`.

## Harness

`build_session` now accepts paths (anything containing `/` is used
as-is), so sessions mix `../stdlib/*.def|mod` with `tests/*`. New
`expect_run_files_out` checks the first output line as well as the
exit code (`Hello` → `Hello, Modula-2!`, `StringsProg` → `Hello
World`).

## Tests

`Hello` (output), `StringsProg` (assign/concat/length/compare →
`Hello World`, `11`, `1`), `t_highlen` → 18; suite 90/90.

## Deferred to 5.3+

`WholeIO`, `Math`, `Files`, `Storage`/`SYSTEM`; explicit external
link names; string escapes; input (`Read`). Traps still `$abort`.