Fix Gantt split-pane ergonomics and defensive mount behavior

[blackopsrepl]

Summary

Clean up implementation-level interaction rough edges in the Gantt wrapper before broader adoption.

Problem

The Gantt wrapper has interaction inconsistencies and weak defensive behavior around mounting and pane management. Those flaws make the component feel fragile in real applications.

Scope

• Review split-pane resize behavior and default ergonomics
• Harden mount and remount behavior when containers are missing, hidden, or resized
• Eliminate brittle assumptions in setup code
• Add tests or demo coverage for the corrected behavior

Acceptance Criteria

• Split-pane interaction behaves predictably across common container sizes
• Invalid or unsupported mount conditions fail clearly instead of crashing unpredictably
• Re-mounting or resizing does not leave the Gantt in a broken visual state
• Tests or fixtures cover the key interaction paths

[blackopsrepl]

Resolved by merged PR #24: https://github.com/SolverForge/solverforge-ui/pull/24\n\nWhat shipped:\n- Gantt mount target validation is now non-destructive\n- remounting recreates chart state correctly instead of leaving a detached/null chart controller\n- split-pane config normalization was hardened while preserving scalar compatibility\n- remount/split lifecycle regression tests were added\n\nClosing this issue because the Gantt mount and split-pane hardening is merged on .

[blackopsrepl]

Resolved by merged PR #24: #24

What shipped:

• Gantt mount target validation is now non-destructive
• remounting recreates chart state correctly instead of leaving a detached/null chart controller
• split-pane config normalization was hardened while preserving scalar splitMinSize compatibility
• remount/split lifecycle regression tests were added

Closing this issue because the Gantt mount and split-pane hardening is merged on main.