# Step: Conversions + RealIO

Tag `v3-stdlib-conversions`. Suite **122/122**; fixpoint **OK**
(`bootstrap/fixpoint.sh`, 2,062,392 bytes).

## Added

- **`Conversions`** — scalar ↔ string:
  - `IntToStr(n, VAR s)`, `CardToStr(n, VAR s)` — decimal, sign for
    negatives, NUL-terminated (written in Modula-2).
  - `RealToStr(x, VAR s)` — 6 significant digits, scientific
    (`%.6E`), via the shim.
  - `StrToInt(s, VAR n):BOOLEAN`, `StrToCard(s, VAR n):BOOLEAN` —
    whole-string parse (optional `+`/`-`), `FALSE` on stray chars or
    empty input.
  - `StrToReal(VAR s, VAR x):BOOLEAN` — `strtod` via the shim;
    `FALSE` unless the whole string is consumed.
- **`RealIO`** — `WriteReal(x)` (no field), `WriteRealF(x, width)`
  (right-justified), `ReadReal(VAR x)` (`scanf("%lf")`).
- **shim** (`runtime/syslib/shim.c`) — `m2realstr`, `m2strreal`,
  `m2writereal`, `m2writerealf`, `m2readreal`; all use the existing
  descriptor convention (`desc[0]` = capacity/length, bytes follow).

## Notes / deviations

- `Conversions` uses `BOOLEAN` results rather than the ISO
  `ConvResults` enumeration (V3 enumeration values in expressions are
  still `230`). Signatures are otherwise "classic" and simple.
- `WriteReal` emits no leading space (consistent with `WriteInt`);
  `WriteRealF(x, w)` pads to `w`.
- The C-ABI match for the existing externals is reused (INTEGER
  formal ↔ C `long`), as in `Files`.

## Tests

- `conv_prog.mod` (exit 127): `IntToStr(-1234)`, `StrToInt("42")`,
  `StrToInt("-7")`, `StrToInt("4x")` → `FALSE`, `StrToCard("99")`,
  `StrToReal("3.5")`.
- `realio_prog.mod` (exit 42): stdout first line `3.500000E+00`,
  `ReadReal` of `1.25` from stdin.

## Files

`stdlib/conversions.def`/`.mod`, `stdlib/realio.def`/`.mod`,
`runtime/syslib/shim.c`, `compiler/tests/conv_prog.mod`,
`compiler/tests/realio_prog.mod`, `compiler/run_tests.sh`,
`docs/plan.md`, `docs/features.md`.