Round number fields to the precision they display

`decimals` only reached `formatNumber`, so it changed the label and nothing else: typing 640.5 into
a field with `decimals: 0` stored 640.5 and displayed 641. The two existing users got away with it
because rounding happened outside the widget (`normalizeFrameRect`, the slice dialog), which their
comments had to point out.

`NumberField.clampValue` now rounds before it clamps, and every path that sets a value goes through
it (typing, scrubbing, arrow keys, the constructor), so the stored number matches the shown one and
`min`/`max` stay exact. The rounding is `roundTo` in math.ts, which `snapToStep` already did inline.

With that safe, camera `View width`/`View height` and text `Font size` become integers – they were
showing 640.000 and 32.000. Everything else that looked integral is not: positions, sprite and
collider sizes and zoom are all scrubbed, and `z` is a summed coordinate (walk.ts), not a layer.

Verified remotely against the RTX 4070 host: 270 checks, all passed (step 10 gained a check that the
field text has no decimal point and survives a reopen). `npm run check` green, 407 tests.

README.md

@@ -404,7 +404,7 @@ npm run format       # Prettier over src, tests and tools
 npm run check        # typecheck + lint + format:check + tests, what CI would run
 npm run build        # writes dist/ and regenerates index.html
 export GM_REMOTE=user@host                                    # where the browser runs; see below
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 267 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 270 editor checks, screenshots in /tmp/gm-e2e-*.png
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs   # 24 checks: the example game, played end to end
 ```
 
@@ -420,7 +420,7 @@ all, so the app never renders.)
 over SSH and forwards its CDP port back to `127.0.0.1`, so the harness itself is unchanged. The
 checks load the public base URL, so the remote side needs only a browser and internet: no repo, no
 node. Setup is an SSH key (`GM_REMOTE_KEY`, default `~/.ssh/gm-win.key`). Measured against a Windows
-desktop with an RTX 4070: **267 checks in 128 s and 24 smoke checks in 16 s, with this machine idle**
+desktop with an RTX 4070: **270 checks in 128 s and 24 smoke checks in 16 s, with this machine idle**
 (0.4 s of CPU for the whole smoke run, load average 0.04). **That is how the suite is meant to be
 run here.** A local run works and costs 124 s, but it pins the development box – set `GM_REMOTE` and
 leave it set; without a reachable host, do not fall back to local, fix the host.