Fix chart popover dimension windowing

[ktsaou]

Summary

• replace duplicated popover window arithmetic with one bounded shared helper
• use the same fixed-size window for line and bar dimension popovers
• cover empty, shorter, equal, longer, missing-selection, and odd-limit boundaries
• add a component regression for a highlighted row in the lower half of a full window

Root cause

The previous end-window calculation could produce a negative start index. With 10 dimensions, a 10-row limit, and the highlighted dimension at sorted index 7, it calculated from = -1; slice(-1, 10) then rendered only the final dimension even though the complete ordered list was unchanged.

The duplicated bar implementation also rounded fractional boundaries outward for odd limits, which could render one row above the configured maximum.

Behavior after this change

• the sorted dimension list remains unchanged for a fixed timestamp
• vertical hover movement only shifts the visible window and its hidden-above/hidden-below counters
• ranges are always non-negative, contiguous, and no larger than the configured limit
• lists at or below the limit are rendered completely

Verification

• yarn test --coverage --collectCoverageFrom=./src/**: 146 suites passed; 1,387 tests passed; 2 skipped
• changed-file ESLint: passed
• CommonJS and ES module production builds: passed
• Chromium against the real app.mem_page_faults stacked chart: the same timestamp and 11-item ordering produced two correct 10-row windows, with exactly one item hidden above or below

Scope

No payload, chart data, sorting, selection, or rendering-engine behavior changes.