Split pane dividers are near-invisible by default (low-contrast darken factor)

[arzafran]

Tracking upstream cmux manaflow-ai#6281 — applicable to our fork but needs a product/scope decision.

Assessment: Confirmed: resolvedSplitDividerColor at Sources/GhosttyConfig.swift:57 darkens a dark background by only 0.40, yielding a near-invisible divider on near-black themes. The fix (raise darken factor or switch to a lightening approach) is mechanical, but it is a default aesthetic change that needs product sign-off on the target contrast value. User-configurable via split-divider-color config key. No regression risk to user-set colors.

Upstream: manaflow-ai#6281

[arzafran]

Decision: Build. One-constant fix: resolvedSplitDividerColor (GhosttyConfig.swift:51-58) darkens dark backgrounds by 0.40 → near-invisible on near-black. Reduce to ~0.15–0.20 (or lighten dark backgrounds instead). User overrides via split-divider-color are unaffected. Cleared needs-decision.

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