fix(promql): count observations with unit sketch weights

[zzylol]

PromQL counts must not vary with sample values. This fixes both count lowering paths and extends the existing frequency-update rule to CMS/CountSketch counts.

Before this PR: count(up) lowered to value cardinality; three series with {1,1,1} expressed a distinct result of 1. count_over_time already lowered to Count, but its CMS/CountSketch plans carried the sample-value column as a weight, allowing value sums and signed CMS updates.

After this PR: count(up) lowers to Count (three vector elements in either {1,1,1} or {1,1,0}), and count sketches carry an explicit item plus constant +1 weight with a UnitCount proof. Ten observations contribute ten updates regardless of whether values are 1, 0, 3, or negative. SQL distinct counting and the existing rejection of unproven signed value-weighted CMS remain unchanged.

Verification: Regression tests failed on the original code for both defects. Added query-text → summary-selection → executable-DAG coverage; corrected existing tests that asserted the erroneous cardinality mapping. All 611 tests passed with cargo +1.98.0 test -p asap-frontend-promql -p asap-aware-mapping --locked. Focused all-target Clippy (-D warnings), workspace formatting, and git diff --check passed. Workspace-wide SQL/integration tests are left to CI.

Reported fixtures: Added numerical planner-contract tests for count(up) on {1,1,1} and {1,1,0} (both 3), ten scrapes of 1/0/3 (each 10), all-negative/mixed-sign series, and CMS update totals for signed/zero values. All five count-planning tests pass, as do focused Clippy and formatting checks. Re-running against the pre-fix implementation makes the new target-count and CMS-total regressions fail. The fixture interpreter covers selected observations and emitted aggregate/update contracts; it is not a backend or sketch-library runtime.

Scope: Planner semantics and executable contracts. Production-process execution was not run; ASAPQuery needs to adopt the fixed revision and replace its hand-built count/HLL oracles as part of ProjectASAP/ASAPQuery-backend#503 and ProjectASAP/ASAPQuery-backend#489. This PR does not claim the broader PromQL rollout is E2E-complete.

Addresses #405.