Bound TAB and SPC padding to a byte

[scottdensmore]

Closes #122.

Problem

TAB and SPC built their padding with strings.Repeat over an unchecked argument, so a single PRINT could allocate arbitrary memory.

$ printf '10 PRINT TAB(200000000);"X"\n' > /tmp/big.bas
$ /usr/bin/time -l ./bin/go-basic /tmp/big.bas > /dev/null
           205881344  maximum resident set size

Fix

Microsoft BASIC takes both arguments as a byte, so the authentic contract is 0 through 255 and a wider request is an illegal quantity. This is a fidelity correction rather than an arbitrary cap.

$ ./bin/go-basic /tmp/big.bas
go-basic: run /tmp/big.bas: BASIC line 10: TAB position cannot exceed 255

$ /usr/bin/time -l ./bin/go-basic /tmp/big.bas > /dev/null
             5177344  maximum resident set size

206 MB → 5.2 MB, which is just the Go runtime baseline.

Two details worth a look

• The range check runs before the int() conversion. int() of an out-of-range float is undefined in Go, so SPC(1E30) would otherwise convert to a platform-dependent value. The obvious ordering — convert, then range-check — is subtly wrong here.
• Truncation toward zero is unchanged. SPC(255.9) still emits 255 spaces. The comparison is argument >= maxPrintPadding+1 for exactly this reason, rather than > maxPrintPadding.

Risk

No pinned corpus program approaches the bound. The widest literal is TAB(60):

grep -rhoiE 'TAB\( *[0-9]+' .cache/basic-computer-games/5301155192d91d74d337899cecc59dbda59c4c17/ \
  | grep -oE '[0-9]+' | sort -n | tail -1   # 60

Computed arguments (TAB(I), TAB(H/12+29), TAB((63-4.5*Y)) are covered by the gate: the smoke tier and the deterministic gameplay tier both pass unchanged, so nothing exceeds 255 at runtime either.

Tests

• TestEvaluatorAcceptsTabAndSpcAtTheByteBoundary — 255 still emits full padding for both.
• tab beyond line width / spc beyond line width rows in TestEvaluatorReportsRuntimeErrors.

Commands run

Full CI gate, green on the committed state:

make corpus-smoke   PASS   all 112 byte-distinct variants passed
make fmt-check      PASS
make vet            PASS
make coverage-check PASS   coverage 83.1% meets 80.0% minimum
make fuzz           PASS
make build          PASS
make release-check  PASS   VERSION=ci
make lint           PASS
make vuln           PASS

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