test: add comprehensive test suite to boost coverage from 63% to ~78%

Add 15 new test files covering previously untested functions across the
package. Tests use local fixtures (no network) and cover core BIDS
project operations, parser internals, plot functions, sidecar/TR
inference, mock BIDS methods, query helpers, project extensions,
confound I/O, pack_bids utilities, and session-aware branches.

Also add .covr_result.rds to .Rbuildignore and .gitignore to prevent
R CMD check NOTE about hidden files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bbuchsbaum

.Rbuildignore

@@ -37,3 +37,4 @@ repomix-output.xml
 ^\.ecosystem\.yml$
 ^AGENTS\.md$
 ^bidser-top5-prd\.json$
+^\.covr_result\.rds$

.gitignore

@@ -20,3 +20,4 @@ docs
 pkgdown/
 CRAN-SUBMISSION
 pybids/
+.covr_result.rds