Write down that the left operand decides integer or float arithmetic

No behaviour change, by decision. `A# * 0.45` is 0 and `0.45 * A#` is 1.35,
because every operator branches on `self->valuetype` and converts the right
operand to match. It is inherited from the Go reference, a C128 promotes to
float instead, and this interpreter is at least consistent about it -- so a
dialect saying the left operand wins is a defensible position, and changing it
to promotion would alter the result of every mixed expression in every existing
program.

**The defect was that nobody said so.** Chapter 3's "Numbers" did not mention
it, Chapter 13 did not list it among the differences, and nothing fails when a
program gets it wrong -- it computes something else and carries on. The game in
examples/ lost its per-level speed increase to `5.6 + LEVEL# * 0.45` evaluating
to a flat 5.6, and bled velocity out of every bounce through `0 - BLVX%(B#)`
quantising to whole pixels. Both read correctly. Neither produced a diagnostic.

Now said in three places: a section in Chapter 3 with the demonstration and the
two rules that keep a program out of it (put the float on the left, put the
answer somewhere with a `%` on it), a row in Chapter 13 naming it as the
difference from 7.0 most likely to turn a working listing into a quietly wrong
one, and the reasoning on value.h where the operators are declared.

tests/value_arithmetic.c pins it in both directions across multiply and
subtract, with a comment saying it is the documented contract rather than an
accident -- so promotion becomes a decision somebody takes deliberately rather
than a change that could slip in under a passing suite.

TODO.md section 9 item 4, struck as a documentation outcome.

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

include/akbasic/value.h

@@ -6,6 +6,16 @@
  * exactly, including the parts that look wrong -- see TODO.md section 12 for the
  * catalogue and the reason they are not fixed yet.
  *
+ * **The left operand decides whether an operation is done in integers or in
+ * floats**, and this is deliberate rather than an oversight: every operator
+ * branches on `self->valuetype` and converts the right operand to match, so an
+ * integer on the left truncates a float on the right. `A# * 0.45` is 0 and
+ * `0.45 * A#` is 1.35. It is inherited from the reference
+ * (basicvalue.go:190-193), a C128 promotes to float instead, and nothing fails
+ * when a program gets it wrong -- it computes something else and carries on.
+ * Documented in docs/03-the-language.md and docs/13-differences.md, and pinned
+ * in tests/value_arithmetic.c so that changing it has to be a decision.
+ *
  * Every binary operator takes a `scratch` value the caller has drawn from an
  * environment's pool and an out-parameter `dest`. Most operators set `*dest` to
  * `scratch` and fill it; akbasic_value_math_plus sets `*dest` to `self` when