Write the usage guide as thirteen chapters in docs/

Organised the way the C128 Programmer's Reference Guide is: the language
first, then each hardware area, then the reference sections. One markdown
file per chapter.

The verb and function references are generated from the interpreter's own
dispatch table, with an assertion that every row is described, so they cannot
drift out of step with what the program accepts. 98 verbs and 30 functions.

Every example was run before it was written down, which caught three claims
that were wrong: a whole FOR loop on one line prints nothing rather than
looping once, MID and INSTR count from zero where a C128 counts from one, and
a multi-line DEF returns a value the caller has to assign away.

Chapter 13 is the list a BASIC 7.0 programmer needs -- roughly sixty
documented differences, including the two known FOR defects and the fact that
drawing does not survive a frame.

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

docs/02-getting-started.md

@@ -0,0 +1,115 @@
+# 2. Getting started
+
+## The prompt
+
+Run `basic` with no arguments and you get a prompt:
+
+```
+$ ./build/basic
+READY
+```
+
+`READY` is printed whenever the interpreter is waiting for you, which is at startup
+and after a program stops.
+
+Anything you type **with a line number** is stored as part of a program. Anything you
+type **without** one runs immediately:
+
+```
+PRINT 2 + 2
+ 4
+READY
+```
+
+## Your first program
+
+```
+10 PRINT "WHAT IS YOUR NAME"
+20 INPUT "> " N$
+30 PRINT "HELLO, " + N$
+RUN
+```
+
+Type `LIST` to see it back, `RUN` to run it again, and `NEW` to throw it away.
+
+## Line numbers
+
+Lines are stored under their numbers and run in numeric order, so the gaps are what
+let you insert later:
+
+```
+10 PRINT "FIRST"
+30 PRINT "THIRD"
+20 PRINT "SECOND"
+LIST
+10 PRINT "FIRST"
+20 PRINT "SECOND"
+30 PRINT "THIRD"
+```
+
+Typing a line number with nothing after it deletes that line. `DELETE 20-40` removes a
+range, and `RENUMBER` tidies the whole program up — it rewrites every `GOTO` and
+`GOSUB` to match, so it will not break your branches.
+
+`AUTO 10` turns on automatic numbering so you do not have to type them; `AUTO 0` turns
+it off again.
+
+## Several statements on one line
+
+Statements are separated by colons:
+
+```
+10 A# = 1 : B# = 2 : PRINT A# + B#
+```
+
+There is one important limit: **block structures do not work inside a single line.**
+
+```
+10 FOR I# = 1 TO 3 : PRINT I# : NEXT I#
+```
+
+prints nothing at all. The loop body is skipped entirely, because the interpreter skips
+forward a *line* at a time looking for the `NEXT` and never finds one on the line it is
+already past. Write loops across several lines:
+
+```
+10 FOR I# = 1 TO 3
+20 PRINT I#
+30 NEXT I#
+```
+
+The same applies to `DO`/`LOOP`.
+
+## Running a file
+
+```sh
+$ ./build/basic program.bas
+```
+
+The file is read, stored, and run. It is exactly the same as typing the program in and
+saving yourself the trouble.
+
+You can also pipe a program in:
+
+```sh
+$ echo '10 PRINT "HI"
+RUN' | ./build/basic
+```
+
+## Saving and loading
+
+```
+DSAVE "myprogram.bas"
+DLOAD "myprogram.bas"
+```
+
+`SAVE` and `LOAD` are the same verbs under their other names. `VERIFY "myprogram.bas"`
+compares what is in memory against the file and prints `OK` if they match.
+
+## Stopping
+
+`QUIT` ends the interpreter. `STOP` stops a *program* and returns you to the prompt,
+where `CONT` resumes it from where it stopped. `END` also stops the program, but does
+not arm `CONT`.
+
+In the SDL build, closing the window stops the program too.