Describe number fields, snap and the scrubbable preview (#248)

Curves.md

@@ -40,12 +40,22 @@ The graph fills the tab, with lines at 0 and 1 in both directions.
 - **Point: …** in the header sets the picked point's type, as in the Vector art tab: *Straight* (no
   handles), *Corner* (handles move on their own), *Smooth* (handles stay in line), *Symmetric*
   (in line and of one length).
+- **t** and **v** in the header show the picked point's place; type a number or drag sideways on
+  the field to move it there. The same rules hold as for a drag, and each change is one undo step.
+  An end's **t** field is off, because the ends stay at 0 and 1.
+- The **magnet** in the header snaps points and handles to a grid of 0.05 in t and v while it is
+  on: a point put on free space, a dragged point and a dragged handle. A point put on the line keeps
+  its place on the line until you drag it.
 - **Preset** starts the curve over from one of the built-in eases — *Sine*, *Quad*, *Cubic*,
   *Quart*, *Quint*, *Expo*, *Circ*, *Back*, *Elastic*, *Bounce*, each *in*, *out* and *in-out*,
   and *Linear*. The menu has a search box. Each preset is traced by a handful of points that stay
   within 0.4 % of the ease, a quad by two, a bounce by twenty, and is yours to bend from there.
-- **Preview** runs a dot along the curve once a second, with a twin on a rail right of the graph
-  that only moves up and down: the motion the curve gives a value. The button holds it.
+- The **preview** is a dot on the curve, with a twin on a rail right of the graph that only moves
+  up and down: the motion the curve gives a value. A playhead runs through the dot, with a knob
+  under the graph. The preview stands still at first; **Preview** runs it once a second, and
+  pressed again holds it. **Drag the knob** sideways to put the preview at any t. **Drag the
+  twin** up or down to jump to the first t at which the curve reaches that height; a height the
+  curve never reaches takes the t where it comes closest.
 - **Fit** frames the curve again, handles included. A middle drag or a Shift drag pans, the wheel
   zooms.
 

Development.md

@@ -15,7 +15,7 @@ npm run build        # writes dist/ and regenerates index.html
 php tools/make-user.php <name> <password> [admin|user]        # an account; every tool signs in (see Accounts)
 php tests/php/accounts.test.php                               # the API's account rules, no server needed (CI's gate job)
 php tests/php/snapshots.test.php                              # the snapshot store (#231), the same way
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1472 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1477 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result