fix: declare pyproj as a direct dependency

[wiyth00]

Summary

chained_eclipse/search.py and chained_eclipse/sensitivity.py both do from pyproj import Geod, but pyproj was never listed in [project] dependencies. It only resolved because cartopy pulls it in transitively. If cartopy ever drops or vendors it, both modules break at import time.

This declares pyproj>=3.6 explicitly and adds a regression test.

• pyproject.toml: add pyproj>=3.6 to [project] dependencies
• tests/test_declared_dependencies.py: imports both affected modules, and asserts pyproj appears in the declared runtime dependencies

CI runs uv sync --all-extras --no-editable without --frozen, so uv.lock is refreshed automatically; a local uv lock before merge is still worth running so the lockfile diff lands in this PR rather than drifting.

Work plan item P0 / 1.

Scientific impact

Packaging metadata only. No solver, ephemeris, geometry, or output code paths are touched, and MODEL_VERSION is unchanged. The 838 km / 3344 kg·m⁻³ second-moon baseline and the 2026-08-12 chained event are unaffected.

• No change to computed results
• Results change (described above)

Checklist

• ruff check . passes
• pytest passes locally
• New or changed behaviour is covered by a test
• Docs / README updated if user-facing behaviour changed

Lint/test boxes left unchecked because this branch was pushed without a local run — CI on this PR is the first execution.