fix: keep Activity panel rows within their width budget (#282)

## Summary

- Follow-up hardening for merged [PR #279](https://github.com/lablup/all-smi/pull/279) and [PR #281](https://github.com/lablup/all-smi/pull/281).
- Preserve PR #279's btop-style height gradient while closing the width-budget gaps left in PR #281's CPU Activity panel regression coverage.
- Guarantee that Activity panel titles, individual core rows, grouped core rows, and narrow gauges remain inside their allocated columns from the minimum supported terminal width upward.

## Target PRs and issues reviewed

- [PR #279](https://github.com/lablup/all-smi/pull/279) / [issue #278](https://github.com/lablup/all-smi/issues/278): the per-row palette mapping, CPU/GPU shared path, annotation isolation, and single-row behavior match the intended implementation. No correctness, security, or performance defect was found; only a stale doc reference to the later-removed left margin needed correction.
- [PR #281](https://github.com/lablup/all-smi/pull/281) / [issue #280](https://github.com/lablup/all-smi/issues/280): the margin removal itself is correct, but the regression fixture explicitly avoided three width paths that still violated the issue's every-line width criterion.

## Review findings

- Trailing partial individual-core rows accounted for a separator after every possible slot instead of the separators actually emitted, making the final line two columns too wide.
- `calculate_cores_per_line` could allocate 15-column gauges even though `draw_bar`'s full representation requires 17 columns, so packing transitions could overflow several times on one row.
- Long socket-group titles were never truncated and escaped the top-border budget at widths near the 81-column minimum.
- PE-cluster and socket-group per-core block sections were unbounded; high-core-count machines could shrink the requested gauge below its real minimum and push the right border far into the neighboring panel.

## Correctness fixes

- Export the full gauge minimum and make `draw_bar` honor smaller requested widths with an exact-width compact fallback.
- Pack individual gauges only when their 17-column representation plus inter-gauge spacing fits, and compute partial-row padding from emitted columns.
- Truncate oversized panel titles with an ellipsis inside the border budget.
- Bound grouped utilization blocks after reserving the full gauge and stop block emission before the panel edge.
- Expand the regression sweep across terminal widths 81 through 200, short and tall graph modes, every collapse strategy, trailing partial rows, and 1 to 128 cores; every row must start at column zero, equal the panel width, and match the reserved panel height.

## Security review

- No authentication, authorization, secret handling, external input execution, filesystem, or network surface is changed.
- Width- and core-count-driven rendering is now bounded by the panel budget, preventing attacker-influenced or malformed high-cardinality telemetry from causing unbounded per-row terminal output.

## Performance and reliability review

- Grouped block rendering now stops when the available columns are exhausted instead of iterating and emitting every reported core on an already-overflowing line.
- The exhaustive width regression completes in the focused test suite without measurable rendering slowdown, and no per-frame heap growth beyond the existing formatting paths was introduced.

## Validation

- [x] `cargo fmt --check`
- [x] `cargo check --lib --tests`
- [x] `cargo clippy --fix --allow-dirty --lib --tests -- -D warnings`
- [x] `cargo clippy --lib --tests -- -D warnings`
- [x] `cargo test --lib ui::activity_panel` (33 passed)
- [x] `cargo test --lib ui::widgets` (15 passed)
- [x] `cargo test` (all unit, integration, and doc-test suites passed)

Refs #278
Refs #280
Refs #279
Refs #281