# Step 8 — VAR actuals with tails (uncommitted)

127/127 tests green (120 + 5 run + 2 rejection; `v_bad_var` repurposed),
mc64 boot + example green, Showcase still 157.

## Goal

Accept `a[i]`, `p^` and record fields as `VAR` actuals (error 233 since
step 4), using the dormant scaffolding (`ActIsVarNext`, per-actual frame
slots). Keep every old suite green with zero behavior change outside
`VAR`-want position.

## What was built

- `MGen`: `ActFrame.addrs[]` (per-actual stashed address temp, -1 when
  unused); `StashAddr` (DUP + temp-store at the Fact tail, where the
  address is complete but not yet loaded — uniform for `[]`, `^` and
  `.` tails); `ClrStash` (invalidates on any value-combining action);
  `ActValue` takes the stashed address for `VAR` formals (address already
  on top for whole array/record actuals, else drop value + reload stash).
- `M2c.atg`: Fact tail stashes when tailed + `ActIsVarNext()` + head is
  `Var/Param/VarPar/Field` + head is not a module + final type is not
  `CHAR`/`BOOLEAN`; `ClrStash()` at every combining site (Rel/AddOp/MulOp
  actions, unary neg, `NOT`, literals, `HIGH`, call results, set elements).
- `run_tests.sh`: step-8 sections added.

## Tests — 127/127 (120 + 5 + 2)

5 run: `v_idx` 35 (`a[2]`, `a[4]`), `v_deref` 105 (`p^.val`),
`v_field` 1031 (`r.b.v`), `v_rec` 16 (whole `r.b` to `VAR` record formal),
`v_row` 21 (matrix rows to `VAR` open-array formal). 3 rejections, each
exactly one error: `v_bad_var` 233 (repurposed: `P(a[i]+1)` — operators
defeat the stash), `v_bad_byte` 233 (`CHAR` elements are byte-packed, no
valid slot address), `v_bad_with` 233 (`WITH`-field actuals).
Bonus finding: row-to-`VAR`-open works, free of charge.

## Bugs found and fixed

1. Stale-stash accept: `P(a[i]+1)` first compiled (stash from `a[i]`)
   because nothing invalidated the address when `+1` followed. Every
   value-combining action now calls `ClrStash()`; direction of error is
   safe (a missed site would only cause a loud 233, and the audit covered
   all `Fact` alternatives, all operator loops, `NOT`, `HIGH`, call
   results and set elements; parens stay transparent).

## Known limits (deferred)

`WITH`-field `VAR` actuals (233), `CHAR`/`BOOLEAN` elements or fields as
`VAR` actuals (233, byte-packing), module vars as `VAR` actuals (233),
type exports, `DEFINITION`/`IMPLEMENTATION` split, module `BEGIN` bodies.