docs: user-facing guides to motion, laser drive and cooling

Two pages aimed at someone who owns the machine rather than works on it,
written for the documentation site. Nothing here is new behavior - it is
the behavior the machine already has, explained where an owner can find
it instead of spread across a kernel contract, a services contract and
three driver headers.

MOTION.md follows one thread: everything physical comes out of a single
fixed-tick byte stream, so the page starts there - the byte layout, speed
as step density rather than clock, the ring and the two ways to fill it,
and the hardware's own stop, halt and resume-with-waypoint. The laser is
presented as part of that stream rather than beside it, which is what
makes the three contract rules (power before fire, no consecutive power
bytes, end dark) and the persisting duty legible instead of arbitrary.
Then geometry and limits, device ownership and the liveness check the
operator sees, and the two modes in full: GRBL from connection through
the arming sequence, the stop/pause/fault table and homing-or-not; cloud
from the preloaded pulse file through the pause backtrack, the park that
ignores the lid, and the ring's cap on job length. A comparison table and
the motion-related settings close it.

COOLING.md explains the engine as what it is - one owner of the thermal
hardware answering a single question for whichever controller runs - and
gives the reasons behind the numbers rather than just the numbers: why
the flow check heats and measures the downstream rise, why 40 percent is
the duty (below it, convection mimics flow), why the settle gate uses a
split-half mean instead of peak-to-peak, and why one bad reading is a
suspicion rather than a fault. Over-temperature, the two diagnostics
tools and when to run them, the settings, and a situation-to-response
table. The fire watch is described honestly: the lid IR channels are
first of all a photometer for the lid lamp, the gate ships watch-only,
and it is not a fire alarm.

Both pages state what is not implemented - low-temperature gates, TEC
control, a fire watch that acts, limit-switch homing - so nobody plans
around them. Constants come from the sources that own them (the feeder
contract, cool.h, the board header, the services contract), not from
prose. README links both.

Documentation only, no behavior change and no catalog consequence: docs/
is outside every layer and .md is excluded from the layer content hash.

README.md

@@ -9,6 +9,8 @@ panel, and a standard Grbl interface.
 * [Installation Instructions](https://github.com/ScottW514/forgefirm/blob/master/INSTALL.md)
 * [Build Instructions](https://github.com/ScottW514/forgefirm/blob/master/BUILD.md)
 * [Connecting LightBurn](https://github.com/ScottW514/forgefirm/blob/master/docs/LIGHTBURN.md)
+* [How motion and the laser are driven](https://github.com/ScottW514/forgefirm/blob/master/docs/MOTION.md)
+* [How cooling and airflow work](https://github.com/ScottW514/forgefirm/blob/master/docs/COOLING.md)
 * [How the laser safing works](https://github.com/ScottW514/forgefirm/blob/master/docs/SAFETY.md)
 * [How a release is accepted](https://github.com/ScottW514/forgefirm/blob/master/docs/ACCEPTANCE.md)
 * [Community Support](https://community.openglow.org)