ci: test optional pandas examples on Linux and Windows

[dev-belly]

The optional CSV adapter proposed in #8 skips eight pandas-dependent tests under the existing default CI installation. Add separate pandas-enabled jobs so that the complete suite and the adapter example are exercised once that contribution is present.

The new matrix covers pandas 2 / Python 3.11 on Linux, and pandas 3 / Python 3.12 on Linux and Windows. Existing default-install jobs continue to verify the dependency-free package. Pandas is not added to runtime dependencies. The example step is conditional because #8 has not merged into main; the full suite always runs.

Validation:

• On the exact docs: add optional pandas CSV input example #8 head (3aa3f9190d51a35cf52073e24359fa58313ef78d), isolated Python 3.12.14 environments passed all 88 tests with pandas 2.3.3 and 3.0.6, with no skips. Their CSV example output was byte-identical.
• A fresh default installation reproduced the coverage gap: 88 tests collected, eight optional tests skipped.
• Both committed scalar and rolling evidence bundles passed independent replay on the adapter tree.
• The current main tree passed all 79 tests locally; workflow YAML parsing and git diff --check passed.
• Windows execution and the new matrix are delegated to this PR's remote CI. Before docs: add optional pandas CSV input example #8 is merged, those jobs validate the existing 79-test tree, not the absent adapter. docs: add optional pandas CSV input example #8's fork workflow still requires GitHub approval.

Remote CI completed successfully on this PR head: all six Linux/Windows jobs passed at https://github.com/dev-belly/PITBridge/actions/runs/37865325374 . These checks cover the current main-based tree; the adapter remains separately proposed in #8. Local review also regenerated both evidence bundles on #8 with pandas 3.0.6: all 12 files matched the committed bundles byte for byte, and compileall passed.