docs(tinybird): update stale Layer 2 descriptions to median imputation (IN-1249)

Signed-off-by: Joana Maia <jmaia@contractor.linuxfoundation.org>

Author: joanagmaia

services/libs/tinybird/datasources/health_score_v2_repo_copy_ds.datasource

@@ -21,9 +21,10 @@ DESCRIPTION >
     - `impactScore` (0-100) — MAX(packages.impact) * 100 over packages published by this repo. NULL when
     the repo has no linked packages.
     - Graceful degradation (spec Layer 1+2): per-category pipes emit NULL when covered sub-signal weight
-    is <40% of that category's max pts (Layer 1); the aggregate health_score_v2 applies Layer 2 rescaling
-    (`SUM(available categories) * (100 / SUM(available category weights))`) so only actually-available
-    scores are summed. If all three categories are unavailable, healthScoreV2 itself is NULL (see
+    is <40% of that category's max pts (Layer 1); the aggregate health_score_v2 imputes each missing
+    category at the population median rate for that category (`missing_max × median(score/max)` over
+    measured repos — IN-1249, 2026-08-28) rather than redistributing the missing weight across measured
+    categories. If all three categories are unavailable, healthScoreV2 itself is NULL (see
     health_score_v2.pipe DESCRIPTION for the full spec).
     - Known temporary gap: security-practices score's spec-stated max is 8 points (includes +1 for
     `security_contact_email IS NOT NULL` flag), but no `security_contact_email` column exists yet in

services/libs/tinybird/pipes/project_insights_copy.pipe

@@ -113,8 +113,8 @@ DESCRIPTION >
     repo in the project," since max-of-maxes = max-over-all.
     - maintainerHealthScoreV2 / securitySupplyChainScoreV2 / developmentActivityScoreV2 (IN-1212): same
     exact-median rollup as healthScoreV2, over the same repo set — kept consistent since healthScoreV2
-    is itself defined as the sum of these three categories at repo level (see
-    health_score_v2_repo_copy_ds). `quantileExact(0.5)` (not quantile/median, which uses reservoir
+    is itself the sum of measured category scores plus population-median imputation for missing categories
+    at repo level (IN-1249; see health_score_v2_repo_copy_ds). `quantileExact(0.5)` (not quantile/median, which uses reservoir
     sampling) already returns NULL, not 0/NaN, when every repo is NULL for a category (confirmed live);
     deterministic output required to reproduce v1↔v2 divergence analysis and acceptance-criteria targets.
 

services/libs/tinybird/pipes/project_repo_insights.pipe

@@ -12,7 +12,7 @@ DESCRIPTION >
     - `page`: Optional integer for pagination offset calculation, defaults to 0
     - Response: Project and repository records with insights metrics
     - `healthScoreV2`, `healthLabel`, `lifecycleLabel`, `impactScore`, `impactLabel` are the Akrites-methodology fields (see `project_insights_copy_ds` for band definitions); may be null when the record has no linked package data. `healthScoreV2` is distinct from the legacy `healthScore` field.
-    - `maintainerHealthScoreV2` (0-40), `securitySupplyChainScoreV2` (0-35), `developmentActivityScoreV2` (0-25) are the Health Score v2 category breakdown (IN-1212). For project records: independent medians (`quantileExact(0.5)`) over the project's enabled, non-excluded repos — they do not necessarily sum to `healthScoreV2`. For repo records: per-repo values passed through from `health_score_v2_repo_copy_ds`; they sum to `healthScoreV2` only when all three categories are available (partial coverage rescales). Null when the record has no value for that category.
+    - `maintainerHealthScoreV2` (0-40), `securitySupplyChainScoreV2` (0-35), `developmentActivityScoreV2` (0-25) are the Health Score v2 category breakdown (IN-1212). For project records: independent medians (`quantileExact(0.5)`) over the project's enabled, non-excluded repos — they do not necessarily sum to `healthScoreV2`. For repo records: per-repo values passed through from `health_score_v2_repo_copy_ds`; they sum to `healthScoreV2` only when all three categories are available; missing categories are imputed at the population median rate (IN-1249). Null when the record has no value for that category.
 
 TAGS "Insights, Widget", "Project", "Repository"