fix: harden graph staging coordination

[jmagar]

Summary

• move graph TEMP staging onto an opaque, dedicated non-pooled SQLite connection
• preserve pooled connection pragmas and serialize persistent projection progress writes
• distinguish writer-admission and write-lock timeout telemetry
• keep pool-error detection structural and improve persistent-failure guidance
• add bounded one-slot-pool concurrency and pragma-parity regressions

Review

Ran code, tests, comments/docs, silent-failure, type-design, and simplification waves until all passes returned clean.

Validation

• cargo fmt --all -- --check
• cargo check --workspace
• cargo clippy --all-targets --all-features --locked -- -D warnings
• cargo test --lib db::pool::tests -- --test-threads=1 (63 passed)
• cargo test --lib db::graph::tests -- --test-threads=1 (35 passed)
• cargo test --lib notifications::dispatcher::tests -- --test-threads=1 (10 passed)