# NUL-terminated string model (self-hosting unblocker)

Tag `v3-nul-strings`. Main suite 111/111.

## Why

The compiler's own sources (and gm2) assume **NUL-terminated**
strings: helpers like `SymTab.Assign`/`Equal` scan `WHILE s[i] # CHR(0)`.
V3 stored `[count, bytes]` with **no terminator** and trapped when
indexing past the count, so those scans aborted. Minimal repro:

```modula2
WHILE (i < HIGH(d)) AND (s[i] # CHR(0)) DO ... END;   (* s[n] out of bounds *)
```

## Change: every CHAR array reserves a terminator slot

- **Model**: a CHAR array of count `n` has `n` data bytes plus one zero
  byte at index `n`; indexed access is allowed for `0 .. n` (the
  terminator is readable/writable). `LEN`/`HIGH` still report `n`/`n-1`.
- **Emission**: `ArrBodyItems` emits `z (n+1)` for CHAR arrays; string
  literals get a trailing `b 0`; `ArrObjSize`/`HeapSize` add 1; a
  heap/local array's terminator is zeroed in `InitHeap`.
- **Bounds**: `Design`'s index handlers use `ArrayHi+1` (fixed) or
  `OpenHiChar` = count (open) for CHAR-element arrays (`SymTab.IsCharArray`).
- **Shim**: `m2write`/`m2fputs`/`m2cstr` derive length by scanning
  (`strnlen`, bounded by the capacity); `m2arg` copies + NUL-terminates
  and **keeps the capacity** in `desc[0]` (it no longer overwrites it
  with the length — that destroyed fixed-buffer capacity).
- **stdlib `Strings`**: rewritten NUL-based (`Length` scans; `Assign`/
  `Concat`/`Delete`/`Append` copy by scanned length bounded by
  `HIGH(dst)` and NUL-terminate).

## Result

- Suite 111/111; the small `NEW` + inline-array-field repro now runs.
- The **self-compiled compiler now runs**: it opens its input (self-
  hosted `FileIO` + shim) and starts parsing.
- It then segfaults in the scanner's `Reset` (`Parse → Reset`) — the
  next runtime bug to chase.

## Note

`desc[0]` is now a **capacity** for arrays (and the literal length for
string literals); string length is derived by scanning. `m2setlen` is
effectively unused.

## Files

`runtime/syslib/shim.c`, `stdlib/strings.mod`,
`compiler/src/{M2.atg,SymTab.def,SymTab.mod,QbeGen.def,QbeGen.mod,M2.lst}`.