fix(control-plane): derive lookback from PromQL

[milindsrivastava1997]

Why

A snapshot's manual time_selection.lookback can disagree with PromQL, causing planning and readout to use the wrong history (#706).

Changes

Derive each query's history from its PromQL tree, including range selectors, subqueries, positive offsets, and every branch of joins, binary operations, and concatenations. Apply required implementation.scrape_interval_ms to each rangeless source before adding enclosing history. Explicit range selectors retain their own materialization windows.

Reject non-null manual snapshot lookback and fractional-second ranges/offsets that the seconds-based backend cannot represent. Migrate discovery output to the new schema and observed scrape cadence, preserve snapshot roundtrips, and add compiler and discovery-to-planner regressions. Resolve conflicts with main while preserving the Min/Max split and current-series execution.

The scrape-cadence default is the backend-local planning contract. It is distinct from Prometheus's instant-selector lookback delta. Current-series execution retains its independent five-minute selector lookback and receives the declared scrape cadence for its input-lag limit. The measured window cost model is preserved; HTTP query_range behavior remains tracked separately in #720.

Before this PR

A 5-minute selector could be planned with a conflicting 1-minute manual lookback. Rangeless queries had no automatic cadence default.

After this PR

A 5-minute selector derives 300 seconds automatically. With a 5-second scrape cadence, sum(a offset 1h) derives 3605 seconds, and a mixed expression containing a 1-second range and a rangeless source retains a 5-second horizon. Histogram concatenations preserve their range requirements. Discovery-generated snapshots deserialize and plan without supplying manual lookback.

Verification

• Established failing regressions for concatenation history, per-source cadence, fractional durations, and discovery schema compatibility before fixing them.
• 416 control-plane unit tests passed on the merged main baseline, including snapshot roundtrip coverage and separate current-series cadence/lookback assertions.
• Discovery-to-planner integration test passed using actual script output and an offset query.
• All 50 o11y-execution Python tests passed.
• Workspace formatting and cargo clippy --workspace --all-targets -- -D warnings passed.
• Compatibility process suite: 15 passed, 1 existing Collector compatibility test ignored; includes current-series, Physical plans for simple repeated PromQL workloads: min_over_time fails to compile; windows ignore query cadence #701/Instant sum/count/avg by and avg_over_time are not accelerated; instant sum fails to compile #702, TopK, and repeated dashboards.
• GitHub MVP backend CI is running on f7a334ba; formatting and workspace check have passed.