Epic 126: add SpatialRust 1.2 streaming release gate

[rsasaki0109]

Summary

• add a fail-closed SpatialRust 1.2 bounded-streaming release gate with typed memory, spill, cleanup, copy, transfer, determinism, and file-handle budgets
• add a canonical measured release example plus Linux/Windows/macOS conformance and feature-isolation CI gates
• publish migration/stability/release documentation, fix prefetch admission under scheduler load, and set workspace/Python versions to 1.2.0

Verification

• cargo test --workspace --all-features
• cargo test -p spatialrust-platform streaming
• cargo run -p spatialrust --no-default-features --features platform,pipeline-streaming --example streaming_1_2_release_gate
• cargo clippy -p spatialrust-platform --all-targets -- -D warnings
• cargo clippy -p spatialrust-records --features receipt-json --all-targets -- -D warnings
• cargo check --manifest-path crates/spatialrust-py/Cargo.toml
• release maturin build plus extracted-wheel Python stream test and 1.2.0 version assertion
• prefetch regression repeated 50 times