Give a loaded line a number when it arrives without one

A script written against LABEL and GOTO NAME never names a line number, so
the numbers it had to carry were decoration. akbasic_runtime_load(),
RUNSTREAM and DLOAD now file an unnumbered line one slot after the last one
filed; a numbered line is filed under its number and moves the cursor, so
the two mix. akbasic_runtime_file_line() is the one implementation of that
rule, so the three paths cannot drift.

The prompt is untouched. A line typed without a number is still direct mode
and still runs now -- that is the only thing separating program text from a
statement at a REPL, and it is why this is a loading feature.

What this replaces was silent data loss: an unnumbered line was filed under
the cursor unchanged, on top of the line before it. A blank line therefore
erased whatever preceded it, and RUNSTREAM did not skip blank lines the way
the other two paths did.

That moves one golden file, and the reference had the same defect.
language/arithmetic/integer.bas has four PRINT statements, an expectation
with three values, and a trailing blank line that erased 40 PRINT 4 - 2
before the program ran. The expectation is now 4 4 2 2.
tests/reference/README.md records the divergence and TODO.md section 5 item
63 says why.

akbasic_SourceLine grows a `numbered` flag so an assigned number can be told
from a written one. RENUMBER sets it on every line it touches; NEW, DELETE
and DLOAD clear it. hadlinenumber moves to akbasic_scanner_scan(), so it
always describes the line just scanned rather than only the REPL's.

Two lines carrying the same written number still keep the last, as they
always have. That is a separate decision.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/reference/README.md

@@ -59,4 +59,5 @@ out as the thing it is.
 
 | Case | Change | Why |
 |---|---|---|
+| `language/arithmetic/integer.txt` | Expectation grew from `4 4 2` to `4 4 2 2` | The program has **four** `PRINT` statements and the expectation had three values. The file ends in a blank line, and both interpreters filed a blank line under the loader's cursor — which for a line with no number of its own is the number of the line before it. So `40 PRINT 4 - 2` was erased before the program ran. The reference lost it the same way, which is why the expectation was written short. Fixed by skipping blank lines in RUNSTREAM, as `akbasic_runtime_load()` and `DLOAD` already did. `TODO.md` §5. |
 | `examples/strreverse.bas` | Variable `INPUT$` renamed to `SOURCE$` | `INPUT$` is a reserved word with a type suffix, which a real C128 refuses and which this interpreter now refuses too (`TODO.md` §6 item 16). The reference accepted it only because its own reserved-word check never fired. The expectation is byte-for-byte unchanged — the program still prints `REVERSED: OLLEH` — so the case keeps all of its value. |

tests/reference/language/arithmetic/integer.txt

@@ -1,3 +1,4 @@
 4
 4
 2
+2