Upgrade to Hydra 0.16

[joshsh]

Currently blocked on Hydra #461. Note also Hydra #442.

[joshsh]

Hydra #442 has landed — the Hydra ↔ TinkerPop bindings are now internalized in Hydra (on main), which unblocks the HydraPop-side shrink this issue tracks.

What's now available upstream in the hydra-pg overlays:

• pg ↔ TinkerGraph bridge — hydra.overlay.java.tinkerpop.coder.HydraGremlinBridge / Validate (Java) and hydra.overlay.python.tinkerpop.coder (Python). These are faithful ports of HydraPop's HydraGremlinBridge.java / Validate.java / gremlin_bridge.py / validate.py (the Java side additionally gains literalToObject, the inverse mapper).
• Gremlin AST ↔ TinkerPop Bytecode mapping — HydraToBytecode / BytecodeToHydra (bidirectional, full step coverage).
• The Modern graph as a translingual built-in sample — hydra.tinkerpop.examples.modern (modernSchema / modernGraph), generated to every host, so HydraPop no longer needs its own ExampleGraphs.

Reciprocal cleanup for HydraPop:

• Delete the vendored bridge/validate sources (src/main/java/.../HydraGremlinBridge.java, Validate.java; src/main/python/hydrapop/gremlin_bridge.py, validate.py) and depend on the internalized versions instead.
• Drop the stopgap decode.py / encode.py — Hydra now generates hydra.encode.pg.model / hydra.decode.pg.model into dist/python.
• Keep the demo-only pieces that were intentionally not internalized: ExampleGraphs.java / GenerateExampleData.java and the validation-scenario JSON under src/gen-main/json/. (These can optionally repoint at the translingual modern sample.)
• Bump the Hydra dependency to 0.17.0 (currently hydra-pg:0.16.1 in build.gradle and hydra-{kernel,pg}>=0.16.1 in pyproject.toml).

One known limitation carried upstream: Gremlin query-text parsing is currently disabled (ANTLR 4.9.1 runtime conflict with the Neo4j Cypher/GQL parser) — tracked as Hydra #521. The bytecode mapping and the pg ↔ TinkerGraph bridge are unaffected.