Add SPC and POS print functions

[scottdensmore]

Closes #121.

Behavior

SPC( and POS( are standard Microsoft 8K BASIC functions that were not reserved words. They lexed as ordinary identifiers, which made them implicit array references that auto-dimensioned and returned 0.

Before

$ printf '10 PRINT "A";SPC(5);"B"\n20 PRINT "AB";: PRINT POS(0)\n' > /tmp/p.bas
$ ./bin/go-basic /tmp/p.bas
A0B
AB0

After

$ ./bin/go-basic /tmp/p.bas
A     B
AB2

Composed with TAB, showing POS reads the same column that TAB and the comma zones write:

$ printf '10 PRINT "COL";TAB(20);POS(0)\n' > /tmp/q.bas
$ ./bin/go-basic /tmp/q.bas
COL                 20

Design notes

• SPC returns a print-list directive rendered as a run of spaces, mirroring how TAB already works. A=SPC(5) is therefore a type error, exactly as A=TAB(5) already was.
• POS returns a plain number, so it works anywhere an expression is allowed, not only inside PRINT. This asymmetry is correct Microsoft behavior and is now stated in the language reference.
• A negative SPC count is a runtime error, mirroring TAB position cannot be negative.
• Both truncate their argument toward zero, consistent with array subscripts.
• POS requires a numeric argument and ignores its value. Microsoft ignores it too; requiring it to be numeric keeps the no-silent-coercion rule.

Risk

No program in the pinned corpus uses SPC or POS — as a function or as a bare identifier:

grep -rnoiE '\b(SPC|POS)\b' .cache/basic-computer-games/5301155192d91d74d337899cecc59dbda59c4c17/   # 0 matches, 210 .bas files

So reserving the names changes no existing transcript. Corpus smoke still passes all 112 byte-distinct variants.

Follow-up filed, not fixed here

#122 — TAB(200000000) allocates 206 MB of padding, measured. That is pre-existing in TAB, and SPC deliberately mirrors it rather than introducing a one-sided guard, so both should be bounded together.

Commands run

Full CI gate, all 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 83.0% meets 80.0% minimum
make fuzz           PASS
make build          PASS
make release-check  PASS   VERSION=ci
make lint           PASS
make vuln           PASS

Tests added: TestLexerRecognizesSpcAndPos, TestEvaluatorEmitsSpcSpacingAndReportsPosColumn (covers SPC(n), SPC(0), POS at column 0, after a semicolon-suppressed PRINT, after a comma zone, and composed with TAB), plus negative spc, spc argument count, and pos argument count rows in TestEvaluatorReportsRuntimeErrors.

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