feat(dash): restore Release/Nightly channel toggle + version pin in onboarding

[michaelroy-amd]

Summary

The dash onboarding's Install ROCm SDK path hardcoded --channel release --format wheel, dropping the Release/Nightly channel choice the legacy tui.rs onboarding used to offer (intentionally dropped when bootstrap adopted the dash onboarding UX in #67). The underlying CLI (rocm install sdk) and the TheRock resolver already fully support --channel, --version, and --build-date (mutually exclusive, validated) — only the onboarding UI was missing. This re-exposes it.

What changed

A new Configure sub-view on the InstallSdk path (crates/rocm-dash-tui/src/ui/onboarding.rs):

• ←/→ (or Tab) toggle channel — Release / Nightly
• ↑/↓ cycle an optional version pin — None / Build date / Version
• typing edits the pin value; Enter stages the install for approval; Esc returns to the menu

The pin maps to the CLI's mutually-exclusive --build-date / --version flags and is available on both channels. A None pin (the default) leaves the command at --channel <ch> --format wheel, so the default Release path is byte-identical to the prior behavior.

Scope is limited to onboarding.rs. No CLI or resolver changes were needed.

Test plan

All in cargo test -p rocm-dash-tui (run --test-threads=1 per the known tui env-race flake):

• Default Release confirm → args unchanged (--channel release --format wheel)
• Nightly toggle → --channel nightly; toggle is reversible
• Nightly + Build date pin → appends --build-date <YYYY-MM-DD>
• Version pin → appends --version <v> (and not --build-date)
• Empty pin value is omitted
• Chars ignored until a pin mode is selected; Backspace edits the value
• Esc returns to the Choose menu
• Existing InstallSdk tests updated for the extra Configure-confirm step
• cargo clippy -p rocm-dash-tui --all-targets clean; cargo build -p rocm green; 531 lib tests pass

Notes

• Independent of the dash-ui-polish-work launcher-routing work; based on main.
• The launcher front-door (bare rocm) does not yet expose this — a separate follow-up if desired.

[rominf]

Approve. The default Release path is provably unchanged (byte-identical args, pinned by a dedicated test), the new Configure sub-view is self-contained and well covered, and the generated commands match the real rocm install sdk contract.

Verified the flag contract end-to-end against the CLI, not just the unit tests:

• --channel release|nightly (default release), --version, --build-date <YYYY-MM-DD> all match — the UI's date placeholder even mirrors the CLI metavar.
• rocm install sdk --channel nightly --build-date <date> parses and reaches the TheRock resolver correctly (a subsequent "no compatible packages for that date" is a data-level result, not a flag issue).
• PinMode being a single enum means the UI can only ever emit one of --version/--build-date, so the mutually-exclusive pair can't be sent together.

Two optional, non-blocking follow-ups:

1. pin_value carries over when the Pin mode is switched (e.g. type a build date, switch to Version, and the date becomes the version value). Clearing it on mode change would re-show the placeholder and avoid feeding stale input to a different flag.
2. Pin values are passed through with no client-side validation (spaces / arbitrary non-control chars accepted); the user only learns of an unusable value after approving and running. Delegating to the resolver is a reasonable scoped choice — just noting there's no pre-flight feedback.

Neither blocks merge.