Bundled bun binary requires AVX — TUI silently fails on non-AVX CPUs (Celeron J4125, etc.)

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

Linux 6.18.36-1-MANJARO - Manjaro Linux

### Expected behaviour

kiro-cli to work

### Actual behaviour

kiro-cli returns to command prompt
kiro-cli --classic works only

### Steps to reproduce

1. open lxterminal
2. kiro-cli

### Environment

```yaml

[q-details]
version = "2.10.0"
hash = "a5da029503141c8033a11d7ee6569583d915cd90"
date = "2026-06-25T20:04:08.842604Z (9d ago)"
variant = "minimal"

[system-info]
chip = "Intel(R) Celeron(R) J4125 CPU @ 2.00GHz"
total-cores = 4
memory = "7.57 GB"

[system-info.os.linux]
kernel_version = "6.18.36-1-MANJARO"
id = "manjaro"
name = "Manjaro Linux"
pretty_name = "Manjaro Linux"
build_id = "rolling"

[environment]
cwd = "/home/USER"
cli-path = "/home/USER"
os = "Linux"
shell-path = "/usr/bin/bash"
shell-version = "5.3.15"
install-method = "unknown"

[env-vars]
DISPLAY = ":0"
PATH = "/home/USER/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/var/lib/snapd/snap/bin:/var/lib/flatpak/exports/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/home/USER/.local/share/gem/ruby/3.0.0/bin"
Q_SET_PARENT_CHECK = "1"
SHELL = "/bin/bash"
TERM = "xterm-256color"
XDG_CURRENT_DESKTOP = "LXDE"
XDG_SESSION_TYPE = "x11"
```

Reproduce the failure with `kiro-cli` on the reported Celeron J4125 system and compare it with `kiro-cli --classic`. Trace how the bundled bun binary is selected or launched, then verify that the normal TUI works on CPUs without AVX while preserving existing behavior on supported CPUs.

Written by the indexing model from the issue text.