system/nxstore: add LVGL touchscreen app-store frontend

[aviralgarg05]

Note: Please adhere to Contributing Guidelines.

Summary

This is another slice of the ongoing GSoC 2026 Dynamic ELF loading and
nxpkg work for NuttX.

An earlier draft PR (#3474) carries the initial nxpkg package
lifecycle helper. PR #3642 extends nxpkg with network sync, install
hardening, and the rest of its CLI (update / remove / rollback /
available). This PR adds the first graphical frontend on top of that:
system/nxstore, an LVGL touchscreen app store.

Series order for this PR:

• base series context: system/nxpkg: add a basic package lifecycle helper #3474 (merged)
• technical dependency: after system/nxpkg: network sync, install hardening, and CLI completion #3642 — this PR's base branch is system/nxpkg: network sync, install hardening, and CLI completion #3642's
  branch (gsoc/nxpkg-sync-lifecycle-hardening-pr5), not master,
  because nxstore calls pkg_sync() / pkg_uninstall() /
  pkg_zalloc() / pkg_free() / pkg_metadata_find_installed() /
  pkg_metadata_find_latest() and reads manifest->description — none
  of which exist yet on master, only as of system/nxpkg: network sync, install hardening, and CLI completion #3642. This PR will not
  build standalone against master until system/nxpkg: network sync, install hardening, and CLI completion #3642 merges; GitHub will
  retarget it automatically at that point and the diff will shrink to
  just this PR's own commit.
• independent of: apps: add missing module metadata for executable tools #3519, examples/sotest: support packaged shared library fixtures #3526, examples/elf: extend nxpkg validation coverage #3531

Concretely, nxstore is:

• a card-based app list (populate_app_list()): each row shows name,
  version, and description/status, install/launch state reflected via
  icon glyph and color, and a sliding-segment progress bar during
  install (no real byte-level progress is available from
  pkg_install(), so this reads as "actively working" without
  fabricating a percentage)
• a supervisor screen (build_run_screen() /
  nxstore_enter_running_screen()): a launched app gets a dedicated
  screen with a name label and a Close button, confined to a border
  region the launched app's own framebuffer output is expected to never
  draw into, so switching back to it doesn't fight over pixels with
  whatever the app already put in the framebuffer
• close/reap handling (close_running_app_event_cb() /
  nxstore_poll_running_app()): sends SIGTERM and polls
  waitpid(WNOHANG), explicitly treating ECHILD as "already gone"
  rather than "still running" (the close-button handler and the passive
  per-loop poll both race to reap the same child). There is
  deliberately no generic force-kill fallback: an earlier version of
  this code called task_delete() when SIGTERM wasn't reaped quickly
  enough, and on real hardware that fallback landed mid
  framebuffer/heap access and hung the entire board, not just the one
  task, on this flat-memory build. A launched app can only be closed if
  it cooperates by installing its own SIGTERM handler
• toast notifications (nxstore_toast()) for install/uninstall/launch
  outcomes and app-closed events, additive to the durable per-row
  subtitle text
• a bounded (15s) wait on DHCP completion before the boot-time catalog
  sync, since Wi-Fi association completing doesn't imply DHCP has —
  fetching before that resolves fails with -ENETUNREACH, which is
  indistinguishable from being genuinely offline without the wait

Impact

User impact:

• adds a new nxstore command: an on-device, touchscreen graphical
  front-end for browsing, installing, launching, and closing nxpkg
  packages, instead of driving the same flow from the nxpkg CLI by
  hand

Build impact:

• adds a new application under system/nxstore
• depends on GRAPHICS_LVGL and SYSTEM_NXPKG (and transitively,
  NETUTILS_CJSON/NETUTILS_WEBCLIENT via system/nxpkg: network sync, install hardening, and CLI completion #3642)

Runtime impact:

• needs a working framebuffer (/dev/fb0) and touch input device
  (/dev/input0)
• spawns launched packages with a fixed 32 KiB stack
  (NXSTORE_LAUNCH_STACKSIZE) — posix_spawn's own default
  (CONFIG_POSIX_SPAWN_DEFAULT_STACKSIZE, 2 KiB) is far too small for
  a real LVGL/framebuffer app and overflowing it does not fail loudly

Compatibility impact:

• none for existing applications unless CONFIG_SYSTEM_NXSTORE is
  enabled

Testing

Host used for build:

• OS: macOS 26.5 arm64
• compiler: xtensa-esp-elf-gcc (crosstool-NG esp-14.2.0_20241119) 14.2.0

Target used for verification:

• arch: xtensa
• board/config used during runtime verification: esp32s3-touch-lcd-7:nsh
• board: Waveshare ESP32-S3-Touch-LCD-7 (custom board directory, not
  yet upstream)

Runtime flow verified on hardware, repeatedly, across the packages this
project has shipped on top of nxstore (a calculator, Conway's Game of
Life, a matching game, snake, brick breaker, and a Doom port — all
separate follow-on PRs):

1. boot to the touchscreen app list, catalog synced automatically at boot
2. tap an uninstalled package's card — install runs in the background
   with a progress indicator, toast on completion
3. tap the now-installed package's card — it launches, and the
   supervisor bar with the Close button appears at the top of the
   screen
4. play/use the launched app for a few seconds
5. tap Close — SIGTERM is sent, the app reaps itself, and the app
   list reappears cleanly

Specifically re-verified in this project's most recent hardware pass,
directly relevant to close_running_app_event_cb() in this PR: launched
a package with a previously-known Close-button regression (its own
rendering was painting over the reserved top bar every frame), tapped
Close, and confirmed via syslog that the sequence in this file executed
correctly end-to-end:

nxstore: close cb fired, active=1 pid=13
nxstore: close SIGTERM sent to 13
nxstore: close reaped=0 gone=1 after 2 tries

This is a draft PR, stacked on the not-yet-merged #3642 — opening now to
get the series in front of reviewers.