Rebuild the launcher and save editor on a small immediate-mode UI kit

The launcher spent ~9ms per frame building and drawing, and the Find Mods
tab could hang the window for minutes. Both had the same root cause: a
retained UI tree rebuilt every frame, and blocking curl calls made from the
draw path.

Replace the vendored FlexLove engine (28.5k lines) with src/ui/kit/ (Kit,
Theme, Layout, Loader). The kit caches Text objects and all measurement,
allocates nothing in the steady state, and draws flat. Build+draw is now
under 1ms at every window size and on every tab (POKEPORT_LAUNCHER_PROF).

Move every network call off the render thread onto a love.thread pool
(src/net/Fetch.lua): mod index fetches, per-mod release checks, find-tab
stats, thumbnails and mod installs. Mod indexes prewarm at boot so the
Find Mods tab is populated before it is opened.

Paginate every list -- mods, find, save slots, settings, release notes,
versions -- with the page size derived from the real viewport height, so a
500-mod index costs what a 10-mod one does. Scrolling is gone.

Anything that waits now raises a non-dismissable loader; per-row background
work shows an inline spinner instead. The in-app updater moves to the top
right beside the settings gear and pulses when an update is waiting.

Theme is black with white outlines, no gradients or glows, and solid
colour-coded embossed buttons with bold labels. The game tabs keep their
cartridge colours. Everything is 1.3x larger. The save editor shares the
theme, and adding an item there is now a searchable pop-up like adding a
Pokemon.

Also:
- Reset rebinds, in Settings and under Touch Controls. Rebinds are additive
  (Input:applyBindings layers them over the defaults), so there was no
  in-game way to undo one.
- Launch options: --game red [--slot N] / POKEPORT_GAME boots straight into
  a game for shortcuts and frontends, falling back to that game's tab when
  its ROM is not imported.

Fixes found while porting:
- Ellipsis and letterspacing truncated bytes, not codepoints, so a
  multi-byte mod name crashed the first frame on a Japanese index.
  Measurement no longer throws on malformed input either.
- The new font set missed UiFont's kana fallback, rendering translated
  builds as tofu.
- Fetch workers idle in Channel:demand() and LOVE waits for live threads at
  exit, so the process outlived the window; quitting mid-download also
  waited on curl's 300s ceiling. Shut the pool down in love.quit and bound
  its transfer timeouts.
- In one column the save-slot card drew below the fold, over the footer,
  with no scrollbar left to reach it.

The two FlexLove engine tests guarded a scroll manager and an auto-height
propagation bug that no longer exist; replace them with a kit suite covering
page bounds, viewport sizing and UTF-8 truncation, and retarget the NX test
to assert the dependency is gone rather than that its perf guards are set.

src/mods/ModIndex.lua

@@ -592,6 +592,101 @@ function ModIndex.fetch(source, opts)
   return index, nil, { fromCache = false, checkedAt = os.time() }
 end
 
+-- ------- async fetch (the launcher's path; ModIndex.fetch above stays as the
+-- synchronous one for tests and non-UI callers)
+--
+-- ModIndex.fetch blocks on curl, which on the render thread froze the Find
+-- Mods tab for as long as the server took.  These three functions are the
+-- same state machine driven a frame at a time over src/net/Fetch.lua:
+--     local h = ModIndex.beginFetch(source, { force = true })
+--     -- every frame:
+--     local done, index, err, meta = ModIndex.pumpFetch(h)
+-- pumpFetch returns done=false while the request is in flight.  The cache
+-- rules are identical to the sync path: a fresh cache short-circuits the
+-- network entirely (so the handle completes on its first pump), a failed
+-- live fetch falls back to stale cache, and the fallback mirror gets one try
+-- before the feed counts as an outage.
+function ModIndex.beginFetch(source, opts)
+  opts = opts or {}
+  local h = { source = source, opts = opts, stage = "start" }
+  if type(source) ~= "table" or type(source.feed) ~= "string" then
+    h.stage, h.err = "done", "missing index source"
+    return h
+  end
+  return h
+end
+
+-- Shared with the sync path's `cached` closure: read whatever is in the
+-- options cache and shape it like a parsed index.
+local function cachedIndex(feed, stale)
+  local entry = ModIndex.readCache(feed)
+  if not entry then return nil end
+  return {
+    schemaVersion = ModIndex.SCHEMA_VERSION,
+    generatedAt = entry.generatedAt,
+    categories = entry.categories or {},
+    mods = entry.mods or {},
+  }, nil, { fromCache = true, stale = stale, checkedAt = entry.checkedAt }
+end
+
+-- Returns done, index, err, meta.
+function ModIndex.pumpFetch(h)
+  if not h then return true, nil, "no handle" end
+  local Fetch = require("src.net.Fetch")
+  local feed = h.source and h.source.feed
+
+  if h.stage == "done" then
+    return true, h.index, h.err, h.meta
+  end
+
+  if h.stage == "start" then
+    if not h.opts.force then
+      local entry = ModIndex.readCache(feed)
+      if ModIndex.cacheFresh(entry) then
+        h.index, h.err, h.meta = cachedIndex(feed, false)
+        h.stage = "done"
+        return true, h.index, h.err, h.meta
+      end
+    end
+    h.job = Fetch.get(feed, { userAgent = "gen1recomp-mod-index" })
+    h.stage = "feed"
+    return false
+  end
+
+  local st = Fetch.poll(h.job)
+  if st.status == "pending" then return false end
+  Fetch.release(h.job)
+
+  if st.status == "ok" and st.body then
+    local index, parseErr = ModIndex.parse(st.body)
+    if index then
+      ModIndex.writeCache(feed, index)
+      h.index, h.meta = index, { fromCache = false, checkedAt = os.time() }
+      h.stage = "done"
+      return true, h.index, nil, h.meta
+    end
+    -- A feed that parses badly is an outage as far as the UI is concerned.
+    h.parseErr = parseErr
+  end
+
+  -- Pages deploys trail a push; the raw mirror is the same file, so a feed
+  -- that fails right after a release is worth one retry elsewhere.
+  if h.stage == "feed" and h.source.fallback then
+    h.job = Fetch.get(h.source.fallback, { userAgent = "gen1recomp-mod-index" })
+    h.stage = "fallback"
+    return false
+  end
+
+  local index, _, meta = cachedIndex(feed, true)
+  h.stage = "done"
+  if index then
+    h.index, h.meta = index, meta
+    return true, index, nil, meta
+  end
+  h.err = h.parseErr or st.err or "index fetch failed"
+  return true, nil, h.err
+end
+
 -- Fetch a description_url / any index-relative text file.  Returns the raw
 -- markdown; callers run it through ModUpdate.cleanBody for display.
 function ModIndex.fetchText(url)