fix(cli): stabilize install progress rendering and download flow

[ian-pascoe]

Summary

• make install progress rendering more reliable by improving phase transitions, finish behavior, and redraw throttling in rich mode
• switch artifact download to an in-process streaming backend with byte-aware progress updates, retries/timeouts, and external fallback controls
• expand CLI tests and install-flow docs to cover progress formatting, backend selection/fallback, and deterministic plain-output contract safety

Test Plan

• rustup run stable cargo fmt --all --check
• rustup run stable cargo clippy -p crosspack-cli --all-targets -- -D warnings
• rustup run stable cargo test -p crosspack-cli

[ian-pascoe]

Heartbeat check (11:35 ET): attempted gh pr update-branch to clear merge conflicts, but GitHub returned "Cannot update PR branch due to conflicts."

Current state: mergeable=CONFLICTING, mergeStateStatus=DIRTY, reviewDecision=REVIEW_REQUIRED.

[ian-pascoe]

Heartbeat check (12:35 ET): re-attempted gh pr update-branch for this PR; GitHub still reports conflicts and cannot auto-update branch.

Current state: mergeable=CONFLICTING, mergeStateStatus=DIRTY, reviewDecision=REVIEW_REQUIRED.

[ian-pascoe]

Heartbeat check (13:35 ET): re-attempted gh pr update-branch; still cannot auto-update because branch conflicts persist.

Current state: mergeable=CONFLICTING, mergeStateStatus=DIRTY, reviewDecision=REVIEW_REQUIRED.