assay-tui: draw_status_bar and draw_help_overlay minor polish (PR #166 review suggestions)

[wollax]

Suggestions surfaced during PR #166 review that were deferred to backlog.

1. draw_status_bar — eliminate redundant .to_string() allocations

Span::raw and Span::styled accept T: Into<Cow<'_, str>>, which &str satisfies directly. The calls to .to_string() allocate needlessly. (The separator-bug fix landed in PR #166 already removed the slug_text allocation; verify project_name is also clean.)

2. draw_status_bar — Style::default().dim() repeated at call sites

The dim style appears multiple times. Extract to a local binding at the top of the function (let dim = Style::default().dim();). This was partially addressed in the PR #166 separator fix (the sep span now uses dim), but verify all remaining call sites are consolidated.

3. draw_help_overlay — document that it must receive frame.area(), not content_area

The function doc comment says it renders on top of all content but doesn't capture the fragile constraint flagged in S05-SUMMARY.md: passing content_area instead of frame.area() would shift the centering up by one row and clip on small terminals. A note was added in PR #166 but may need strengthening if the call site is ever refactored.

Suggested inline note (already added in PR #166 — confirm it survived):

/// NOTE: must be called with `frame.area()` (full terminal rect), *not* with
/// `content_area`. The popup uses full-screen geometry to center correctly …

4. handle_event help-overlay guard — clarify that q is also consumed

The guard comment says "all other keys are no-ops" but doesn't call out the most surprising consequence: q does not quit while the overlay is open. A parenthetical would make this explicit for future maintainers debugging unexpected quit behaviour.

// When help overlay is visible, only ? and Esc dismiss it; all other keys —
// including q (quit) — are consumed as no-ops so nothing fires through the
// overlay by accident.

5. draw_status_bar doc comment — document dim-styling contract

The current doc describes the layout but not which spans are styled. project_name renders at default brightness; everything else is dim. Add one line:

/// `project_name` renders at default brightness; the separators, `cycle_slug`,
/// and key hints are all rendered dim.

---

All of these are cosmetic / doc improvements. No behaviour change required. Safe to batch into the next assay-tui polish pass.