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/04-control-flow.md

@@ -183,6 +183,33 @@ ARRIVED
 using for its own sake: a program written with labels is immune to `RENUMBER`, because
 there is no number to rewrite.
 
+Follow that one step further and the numbers stop earning their keep at all. A program
+in a file can leave them out — see
+[Chapter 2](02-getting-started.md#a-program-in-a-file-does-not-need-them) — and then
+every branch it makes is by name:
+
+```basic
+GOSUB GREET
+GOTO FINISH
+
+PRINT "NOT REACHED"
+
+LABEL GREET
+PRINT "HELLO"
+RETURN
+
+LABEL FINISH
+PRINT "DONE"
+```
+
+```output
+HELLO
+DONE
+```
+
+Nothing in that program can be broken by inserting a line into it, which is the whole
+argument.
+
 ## ON
 
 `ON` picks the *n*th target from a list, counting from one: