# Step: UString concat/LEN + IOChan position/seek

Tag `v3-polish-ustr-iochan`. Suite **137/137**; fixpoint **OK**
(image **2,156,095 bytes**).

Two of the three remaining polish items; Clarion `CLASS` lowering is
scoped separately (`docs/summary_class-lowering.md`).

## UString concatenation and length

- `U"a" + U"b"` concatenates descriptors: `QbeGen.UStrCat` →
  shim `m2ustrcat` (static buffer; consume before the next concat).
- A **single-codepoint** operand (`U"x"` is a `UCHAR`) becomes a
  1-codepoint UString via `QbeGen.UStrFrom` → `m2ustrfrom`. The shim
  uses a small rotating pool so an expression can supply both
  operands before `m2ustrcat` reads them.
- `LEN`/`LENGTH`/`HIGH` of a `U"…"` literal read the descriptor
  header: `QbeGen.UStrLen` → `m2ustrlen` (and `DecQ` for `HIGH`).
  A single-codepoint literal is `LEN = 1`, `HIGH = 0`.
- `+` handles `UString+UString`, `UString+UCHAR`, `UCHAR+UString` and
  `UCHAR+UCHAR`; the result type is `ClUStr`.

Note: `LEN(expr)` where the operand is a computed expression (e.g.
`LEN(U"a" + U"b")`) is not accepted — `LEN`/`HIGH` take a designator
or a literal, as before.

## IOChan positioning

- `Position(cid):INTEGER`, `Seek(cid, pos):BOOLEAN`,
  `Rewind(cid):BOOLEAN`.
- Over `SysShim.ftell`/`fseek` → shim `m2ftell`/`m2fseek`
  (`fseek(..., SEEK_SET)` + `clearerr` on rewind).

## Tests

- `t_ustrcat.mod` (63): `LEN`/`LENGTH`/`HIGH` of literals,
  `U"ab"+U"cd"`, `U"x"+U"é"`.
- `iochanseek_prog.mod` (15): write 6 bytes, `Position`, reopen,
  `Position` after reads, `Rewind`, `Seek(3)`, read line back.

## Files

`compiler/src/{QbeGen.def,QbeGen.mod}` (`UStrCat`/`UStrFrom`/
`UStrLen`/`DecQ`), `compiler/src/{SymTab.def,SymTab.mod}`
(`UStrCompat`), `compiler/src/M2.atg` (UString `+` and `LEN`),
`runtime/syslib/shim.c` (`m2ustrcat`/`m2ustrfrom`/`m2ustrlen`/
`m2ftell`/`m2fseek`), `runtime/syslib/SysShim.def`/`.mod`,
`stdlib/iochan.def`/`.mod`, `compiler/tests/{t_ustrcat,iochanseek_prog}.mod`,
`compiler/run_tests.sh`, `docs/features.md`.