examples/fbcon: Do not require a builtin registry to spawn the shell.

[Fishwaldo]

Summary

• examples/fbcon does not build in a kernel build. It asks the registry
  of built-in applications for the stack size and priority to spawn its
  shell with, and a kernel build has no such registry, so the reference
  does not resolve.
• A kernel build's programs are ELF files in a filesystem, which is
  exactly what the posix_spawn() below already handles, PATH search and
  all. There is nothing for the registry to tell it.
• fbcon now consults the registry only where there is one, and takes the
  stack size and priority from this example's own configuration otherwise.
• Companion to import: Link libnx for graphics applications in a kernel build. #3707, which puts libnx on the kernel-build
  link line. Both are needed for fbcon specifically; this one is fbcon's
  own defect, that one affects every NX application. Neither depends on the
  other to be correct, but fbcon needs both to link.
• No related issue filed.

Impact

• Is new feature added? Is existing feature changed? NO. Build fix.
• Impact on user? YES, positive, and confined to this example. Where a
  builtin registry exists the behaviour is byte-for-byte what it was; the
  new path is reached only where the registry does not exist, which is
  where fbcon previously did not compile.
• Impact on build? NO beyond fbcon compiling where it did not.
• Impact on hardware? NO.
• Impact on documentation? NO.
• Impact on security? NO.
• Impact on compatibility? NO.
• Build-mode dependence? YES, CONFIG_BUILD_KERNEL only, in the sense
  that the guarded path is reached only where CONFIG_BUILTIN is unset,
  which is the kernel-build case. Flat and protected builds keep the
  registry lookup unchanged.

Testing

I confirm that changes are verified on local setup and works as intended:

• Build Host: macOS 26.5.1, arm64 (Apple Silicon), xPack riscv-none-elf-gcc
  15.2.0
• Target: RISC-V, ESWIN EIC7700X EVB (downstream board port, not yet
  upstream), kernel build, HDMI framebuffer at 1080p60

Testing logs before change, linking examples/fbcon with
#3707 already applied so that libnx is no longer the
obstacle:

/Users/fish/github/nuttx/libs/libc/builtin/lib_builtin_forindex.c:56:(.text.builtin_for_index+0x0): undefined reference to `g_builtin_count'
riscv-none-elf-ld: /Users/fish/github/nuttx/libs/libc/builtin/lib_builtin_forindex.c:58:(.text.builtin_for_index+0x12): undefined reference to `g_builtins'
make[3]: *** [/Users/fish/github/apps/bin/fbcon] Error 1

Testing logs after change: fbcon builds, starts, and renders its console on
the framebuffer, spawning a shell whose prompt appears on the monitor:

[CPU0] display: /dev/fb0 1080p60
##### CMD 1: fbcon &
fbcon [0:100]
##### END 1 (ok, 1.57s)

With glyph rendering traced, showing it drawing to the framebuffer:

##### CMD 1: fbcon
[CPU2] fbcon_renderglyph: ch=H [48]
[CPU2] fbcon_renderglyph: ch=e [65]
[CPU2] fbcon_renderglyph: ch=l [6c]
[CPU2] fbcon_renderglyph: ch=o [6f]

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation.
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

---

Claude (claude-opus-5) assisted with diagnosing this and with authoring the
code comment and this PR description. The commit carries an Assisted-by: tag
per CONTRIBUTING.md §1.5.