fix(settings): order parseSettings before initBuckets; dedupe color defaults

[Demonkratiy]

Summary

Small follow-up cleanups after #221 (gradient middle feature).

Changes

• Run parseSettings() before initBuckets() in update(). parseSettings() can flip enableColorbrewer / activateGradientMiddle (e.g. in high-contrast mode), so running it first ensures bucket constraints (buckets.options.min/maxValue, CurrentBucketCount) match the final rendered mode instead of being stuck at the pre-parse Colorbrewer limits.
• Dedupe stroke / textColor defaults into GeneralSettings.DefaultStroke / GeneralSettings.DefaultTextColor constants, referenced from both GeneralSettings and parseSettings() to avoid future drift.
• Add a high-contrast regression test that drives the full update() flow and asserts the post-parse bucket constraints (minValue = 1, maxValue = 18). This test fails on the old initBuckets → parseSettings order and passes on the new one, guarding the fix against future reordering.

Notes

• Out of scope: the start/end Colorbrewer write-back rework (single-source-of-truth color sync via persistProperties) will be a separate PR.

Validation

• eslint src/ clean
• 71/71 unit tests pass
• Regression test verified to fail on the previous ordering and pass on the new one

[Copilot]

Pull request overview

This PR is a focused follow-up to the gradient-middle work (#221) that fixes a settings initialization ordering issue affecting high-contrast rendering, and centralizes a couple of default color constants to avoid drift.

Changes:

• Reorders update() to run parseSettings() before initBuckets() so bucket min/max constraints are computed from the final (post-parse) mode, particularly in high-contrast mode.
• Deduplicates default stroke and textColor values into GeneralSettings.DefaultStroke / GeneralSettings.DefaultTextColor and reuses them in both settings initialization and parseSettings().
• Adds a high-contrast regression test that drives the full update() flow and asserts bucket constraint min/max values match the non-colorbrewer limits.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/visualTest.ts	Adds a high-contrast regression test validating post-parse bucket min/max constraints.
src/visual.ts	Reorders settings initialization so parseSettings() runs before initBuckets().
src/settings.ts	Introduces shared constants for default stroke/text colors and uses them in GeneralSettings.
src/heatmapUtils.ts	Uses GeneralSettings default color constants in parseSettings() to avoid duplicated literals.

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