fix: honour flex-separator in powerline render path

[ssoriche]

Summary

renderPowerlineStatusLine filters out flex-separator widgets at the top of the function and never restores their layout effect, so a configured flex-separator silently has no effect when powerline mode is enabled. The non-powerline render path correctly distributes terminal-width minus content-width across each flex-separator position; this PR teaches the powerline path to do the same.

Repro

In an existing config, enable powerline.enabled: true and place a flex-separator widget between two regular widgets. Render against a known terminal width and observe the visible width of the result is content-width, not terminal-width — the flex space is missing.

Fix

In src/utils/renderer.ts, inside renderPowerlineStatusLine:

1. Track flex positions before filtering. Build a Set<number> of filteredWidgets indices that are immediately followed by a flex-separator in the original widgets array. The widget itself stays filtered (powerline can't render it as a triangle), but the layout intent is preserved.
2. Emit a sentinel + section cap during the render loop. When a flex position is reached, emit a closing powerline cap — a triangle in the previous widget's bg colour with no bg of its own, so it terminates the left section visually — followed by a �FLEX_SEP� sentinel. Skip the regular between-widgets separator at that boundary.
3. Post-process after the loop. If any flex positions were marked, split the result on the sentinel, measure each part's visible width, distribute terminalWidth − totalContentWidth evenly across the flex positions as spaces, and reassemble. When the terminal width is unknown the sentinels are stripped so they can't leak into the output.

The non-powerline path is unchanged.

Test plan

Four new tests in src/utils/__tests__/renderer-flex-width.test.ts:

• distributes remaining width across a flex-separator in powerline mode — single flex, asserts visible width equals the effective render width (terminalWidth − 6 for flexMode: 'full').

• distributes space across multiple flex-separators in powerline mode — three sections separated by two flex-separators, total visible width still matches effective width.

• strips flex-separator markers when terminal width is unknown — sentinel never leaks into output when no width is detected.

• still works in non-powerline mode (no regression) — sanity check on the existing path.

• All 585 tests pass (bun test) — 581 existing + 4 new

• TypeScript checks clean (bun tsc --noEmit)

• ESLint clean (bun run lint)

• Manually verified in a real Claude Code session (visible width fills to terminalWidth − 6 as expected)