fix: use explicit namespace for statmod::dinvgauss in wald/pdf runner

Replace `library( statmod )` with explicit `statmod::dinvgauss()` call
to resolve `object_usage_linter` error in CI, which cannot see the
function definition loaded via `library()`.

Assisted-by: Claude Opus 4.6 <noreply@anthropic.com>

---
type: pre_commit_static_analysis_report
description: Results of running static analysis checks when committing changes.
report:
  - task: lint_filenames
    status: passed
  - task: lint_editorconfig
    status: passed
  - task: lint_markdown
    status: na
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    status: na
  - task: lint_repl_help
    status: na
  - task: lint_javascript_src
    status: na
  - task: lint_javascript_cli
    status: na
  - task: lint_javascript_examples
    status: na
  - task: lint_javascript_tests
    status: na
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    status: na
  - task: lint_python
    status: na
  - task: lint_r
    status: passed
  - task: lint_c_src
    status: na
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    status: na
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    status: na
  - task: lint_c_tests_fixtures
    status: na
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  - task: lint_typescript_declarations
    status: passed
  - task: lint_typescript_tests
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  - task: lint_license_headers
    status: passed
---

Author: batpigandme

lib/node_modules/@stdlib/stats/base/dists/wald/pdf/test/fixtures/r/runner.R

@@ -79,15 +79,12 @@ main <- function() {
 	# Get the directory of this script:
 	source_dir <- dirname( get_script_path() );
 
-	# Load the statmod library for dinvgauss:
-	library( statmod ); # nolint
-
 	# Generate test fixture data:
 	# Note: x, mu, and lambda must be > 0 for Wald distribution
 	mu <- sample( seq( 0.1, 10.0, length.out = 1000L ) );
 	lambda <- sample( seq( 0.1, 100.0, length.out = 1000L ) );
 	x <- sample( seq( 0.01, 100.0, length.out = 1000L ) );
-	y <- dinvgauss( x, mu, lambda );
+	y <- statmod::dinvgauss( x, mu, lambda );
 
 	data <- list( mu = mu, lambda = lambda, x = x, expected = y );