fix(behavior): use viewport filtering after the first render

[dvd233]

Summary

• Clear AutoAdaptLabel's first-render guard after the initial full label scan.
• Use viewport-filtered label candidates for subsequent transforms and redraws.
• Add a regression test covering the post-render zoom path.

Why

isFirstRender was initialized to true but never reset, so getLabelElementsInView() was unreachable. Every later label recomputation therefore continued to sort and collision-check the full element map, even after the graph had been zoomed into a smaller viewport.

The initial full scan is preserved for first-render correctness; later updates now use the existing viewport-filtered path.

Fixes #7675

Validation

• pnpm --filter @antv/g6 exec jest __tests__/unit/behaviors/auto-adapt-label.spec.ts --runInBand -t "uses viewport filtering after the first render" ✅
• pnpm --filter @antv/g6 type-check ✅
• pnpm --filter @antv/g6 lint ✅
• git diff --check ✅

The complete existing auto-adapt-label.spec.ts suite still has four pre-existing SVG snapshot failures in this environment (expected: undefined, received: undefined); the new regression test passes. The snapshot failures reproduce on the unmodified baseline.

Codex was used to prepare the patch; the diff and validation results were reviewed before submission.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.46%. Comparing base (c4fc050) to head (86e81c1).
⚠️ Report is 2 commits behind head on v5.

Additional details and impacted files

@@            Coverage Diff             @@
##               v5    #7682      +/-   ##
==========================================
+ Coverage   94.43%   94.46%   +0.03%     
==========================================
  Files         188      188              
  Lines       10099    10100       +1     
  Branches     2201     2201              
==========================================
+ Hits         9537     9541       +4     
- Misses        516      559      +43     
+ Partials       46        0      -46     

Flag	Coverage Δ
g6	94.46% <100.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
packages/g6/src/behaviors/auto-adapt-label.ts	96.21% <100.00%> (+2.31%)	⬆️

... and 15 files with indirect coverage changes

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