Document optional line numbers

Chapter 2 gets the rule and the refusal, chapter 4 gets the payoff for
LABEL, chapter 9 says DSAVE writes the numbers it handed out and to
RENUMBER first if you want gaps, chapter 10 shows a host loading numberless
source, chapter 13 gets the QuickBASIC-shaped divergence, and chapter 14's
source[] passage gets its second half.

Chapter 14 said "two prescans" and listed two; there were three before this
and there are four now, so it lists all four and says which of them reports
against the right line.

TODO.md section 6 records four things found on the way and deliberately not
fixed: set_label() filing into the active scope rather than the root, three
prescans reporting the wrong line number, duplicate written line numbers
still replacing silently, and renumber.c's file-scope scratch arrays.

examples/embed.c runs the same program twice, numbered and not, so the
example compiles the feature rather than describing it. Its header pointed
at ./build/examples/embed, which is not where the binary lands.

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

docs/13-differences.md

@@ -92,6 +92,29 @@ A statement typed with no line number runs immediately, as it should. This was n
 true until recently — the interpreter used to file everything but a handful of verbs as
 program text.
 
+## Line numbers
+
+**A program in a file does not need them.** Every BASIC this dialect descends from
+required a number on every line; here that requirement belongs to the prompt alone,
+where the number is the only thing separating program text from a statement to run now.
+A program loaded from a file, or handed to the library as a string, may leave them out,
+and a line without one is given the next number going. The two mix: a numbered line sets
+where the next unnumbered one goes.
+
+This is QuickBASIC's idea rather than the C128's, and it is here for the same reason
+QuickBASIC had it — a program that branches by `LABEL` never names a line number, so the
+numbers are maintenance with nothing on the other end of it.
+
+Two consequences worth knowing:
+
+- **`GOTO <number>` must name a number the program wrote.** In a file with no line
+  numbers `GOTO 100` would otherwise find the hundredth line and branch there. It is
+  refused before the program runs. `GOTO <label>` is unaffected.
+- **`LIST` and `DSAVE` show the numbers that were handed out**, one apart. `RENUMBER`
+  before `DSAVE` if you want gaps to insert into.
+
+An unnumbered program is capped at 9998 lines, which is the cap that already applied.
+
 ## Errors
 
 `ER` and `EL` are **`ER#` and `EL#`**, ordinary global variables. `ER#` holds this