A string that spells a directive is a string, not a directive

The quotes are gone by the time a token is classified, so checkIfKeyword's
test of token[0] == '#' matched the STRING "#Program" sitting in a program's
Data Segment. It was read as the directive: the segment silently changed in
the middle of the data, the string's nine bytes were charged to the Program
cursor instead of the Data one, and every label defined after it came out
nine bytes wrong - in a file that still had a valid header, a plausible
length, and nothing to say about any of it. The only symptom was a program
that jumped into the middle of an instruction.

This is the FOURTH of the family. A string spelling a mnemonic assembled as
that instruction; a string beginning with a zero was rejected as a malformed
literal; a string in the Program Segment was discarded in silence. The
instruction check and the literal check both carry a "not a STRING" guard
already. This one did not, so it has one now, and it lives inside
checkIfKeyword rather than at the call site so it cannot be left off again.

Nothing had ever triggered it, because nothing had ever needed a directive's
name as data. An assembler written FOR this machine necessarily does: it has
to compare tokens against "#Program" and "#Data". It was found by building
one and watching it fault on its second instruction.

Test stringKeyword puts every directive name in a program's data and prints
a label defined after them. Verified that it bites: without the fix the
assembler refuses the file outright.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Tests/expected/stringKeyword.out

@@ -0,0 +1,4 @@
+#Program #Data #Include #Vectors #Base #Align #Reserve
+and a label after them still points at itself
+Execution halted.
+[exit 0]

Tests/manifest

@@ -387,6 +387,11 @@ diagUnbasedSegment   | testPrograms/diagnostics/unbasedSegment.asm | xfail | -
 lib-print            | Libraries/print.asm                | xfail    | -                  | -
 # These three call print.asm routines but have no #Include line at all. They are
 # only ever pulled in by printTest.asm, so they are not standalone programs.
+# A string that spells a directive is a string. The quotes are gone by the time a token is
+# looked at, so "#Program" in a program's data was read as the directive: the segment
+# changed in the middle of the Data Segment and every label after it came out nine bytes
+# wrong, in a file that still had a valid header and a plausible length.
+stringKeyword        | testPrograms/stringKeyword.asm     | run      | -                  | -
 printDecimalTest     | testPrograms/printDecimalTest.asm  | xfail    | -                  | -
 printDigitTest       | testPrograms/printDigitTest.asm    | xfail    | -                  | -
 printHexTest         | testPrograms/printHexTest.asm      | xfail    | -                  | -