fix: clamp vertical marker stacks at the canvas edge

[ianlapham]

What

• A "vertical" marker column is now also capped where the next glyph would cross the canvas edge, not just at maxVisible.
• Stacks anchored near the top or bottom of the plot hide their off-canvas overflow (same hidden treatment as maxVisible); the base glyph always draws.

Bugs fixed

• Vertical stacks climbed off the plot when their anchor sat near the canvas edge — maxVisible caps the glyph count but not the geometry, so a short cap on a tall chart still let edge-anchored columns escape.

Why bounds live on ClusterMarkersOpts, not MarkerClusterConfig

• The canvas edge is call-site geometry, not user preference: the three internal callers pass 0 / canvasHeight, the clamp is always-on, and ClusterMarkersOpts stays unexported — no public API change.

Why canvas bounds, not plot-area bounds

• Bounds are [0, canvasHeight], so a stack may still enter the axis gutters — glyphs stay fully visible, which is the bug being fixed; clamping to the padded plot rect would hide more than necessary.

Why the base slot always draws (cap floor of 1)

• When the anchor itself sits at or past a bound the fit math goes negative; flooring at 1 keeps the marker visible at its anchor instead of vanishing the whole bucket.

Test plan

• npm run verify green (typecheck, lint, 99 suites / 1502 tests); the pre-commit hook re-ran the tests on commit.
• New markerCluster tests: fit math for above and below columns, cap floor of 1 with negative room, min(maxVisible, bounds cap) in both directions, pass-through when a bound is missing, and the step-0 (overlap: 1) guard. Existing tests pass unmodified (bounds omitted = behavior-identical).
• Not re-verified visually in the example app here; this exact behavior has been running in production at FOMO as a carried patch.

🤖 Generated with Claude Code

[brandtnewlabs]

Blocker resolved

Merged current main into this branch in 4e6f09d and resolved the only conflict in CHANGELOG.md by retaining both the vertical-marker-stack fix and the already-merged tail-less-badge fix.

The integrated tree was revalidated before pushing:

• focused marker math/overlay/hit-test suites: 44 tests passed
• npm run verify: passed against current main
• React Doctor: 96/100; only the pre-existing large-component warning
• agent-device QA on iPhone 17 Pro:
  • built-in and custom vertical stacks stop at both canvas edges
  • hidden overflow remains hidden
  • maxVisible: 4 remains the tighter cap
  • clean runtime logs after a full reload

No functional review findings remain.