Harden priority geospatial compatibility paths

[Claudio9701]

Outcome

Closes the highest-priority geospatial compatibility backlog with regression coverage against the currently locked ecosystem.

Fixes

• proves H3 4.x hex generation uses the current API and returns unique CRS-aware cells
• proves OSMnx 2.x nearest-node calls use longitude then latitude
• keeps graph isochrones in the graph CRS, avoids mutating the caller's graph, and uses standard OSMnx x/y node coordinates
• makes merge_shape_hex safely rerunnable on its own result by preventing aggregate-column suffix collisions
• makes one-item and multi-item HDX selections deterministic (the one-item list caused the reported invalid buffer error)
• locks the current HDX client, bounds exact searches, provides stable provider errors, and handles empty/missing resources explicitly
• preserves CRS through accessibility buffers, pressure maps, and outputs
• updates deprecated GeoPandas op= spatial joins to predicate= and fixes already-projected pressure-map inputs

Verification

• pytest -q: 56 passed, 2 deselected
• warning-as-error Sphinx build: clean
• Ruff: clean
• Trunk: 115 files, no issues
• live read-only HDX contract: Peru search returned 7 CSV resources with the locked hdx-python-api 6.6.8 client

Stack

Base: #84
Roadmap: #61
Parent: #70

Closes #14
Closes #19
Closes #20
Closes #27
Closes #38
Closes #52