Implement Workstream Health Index (WHI) KPI card

Add a live WHI card to the Workstreams page TOC sidebar. All six base
metrics from the spec (workstream-kpi.md) computed client-side from
existing data — no API or schema changes required.

Computation (workstreams.md):
- DD: dependency edges / open workstreams (normalised at DD_crit=1.0)
- BR: blocked workstreams / open workstreams
- SPR: max inbound deps on one incomplete workstream / open count
- PEP: active workstreams with all deps completed / open count
- CDDR: cross-domain edges / total edges
- CPI: DFS cycle detection (back-edge = 1, halves WHI as hard penalty)
- WHI = 0.30(1-DDnorm) + 0.25(1-BR) + 0.15(1-SPR) + 0.20·PEP + 0.10(1-CDDR)
- Per-domain breakdown using intra-domain edges only

Card UI: global WHI % with green/orange/red health label, sub-metric
rows with per-spec warning thresholds, cycle alert panel, per-domain
breakdown rows with coloured dots.

Also add src/docs/workstream-health-index.md reference page (formula,
thresholds, improvement guidance) and wire ? button on the card.
Add "Workstream Health" to Reference nav.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dashboard/src/docs/workstream-health-index.md

@@ -0,0 +1,161 @@
+---
+title: Workstream Health Index — Reference
+---
+
+# Workstream Health Index (WHI)
+
+The **Workstream Health Index** is a composite score in the range [0, 1] that measures how well the workstream network is structured for parallel execution and stable progress. It is displayed as a live KPI card in the right margin of the Workstreams page and recomputes on every poll (every 15 seconds).
+
+**1.0 = ideal independence · 0.0 = severe systemic dysfunction**
+
+---
+
+## Health states
+
+| Score | Color | Label | Meaning |
+|---|---|---|---|
+| ≥ 0.75 | 🟢 green | Healthy | Parallel execution effective, delays localized |
+| 0.50 – 0.74 | 🟠 orange | Optimizable | Noticeable coordination cost; review decomposition |
+| < 0.50 | 🔴 red | Critical | Serial execution dominates; immediate replanning required |
+
+---
+
+## The six base metrics
+
+### DD — Dependency Density
+
+```
+DD = total dependency edges / (active + blocked workstreams)
+```
+
+Measures structural coupling. Low DD means independent, parallelizable work. Completed and archived workstreams are excluded — they no longer constrain progress.
+
+| DD | Warning |
+|---|---|
+| > 1.0 | 🔴 red — more than one dependency per workstream on average |
+| 0.5 – 1.0 | 🟠 orange |
+| ≤ 0.5 | ok |
+
+---
+
+### BR — Blocked Ratio
+
+```
+BR = blocked workstreams / (active + blocked workstreams)
+```
+
+Measures immediate operational impact. BR ≈ 0 means flow is unobstructed.
+
+| BR | Warning |
+|---|---|
+| > 40% | 🔴 red |
+| 20–40% | 🟠 orange |
+| ≤ 20% | ok |
+
+---
+
+### SPR — Single-Point Risk
+
+```
+SPR = max dependents on one incomplete workstream / (active + blocked)
+```
+
+Detects concentration of blocking power. High SPR means one delay propagates widely — a structural SPOF.
+
+| SPR | Warning |
+|---|---|
+| > 40% | 🔴 red |
+| 25–40% | 🟠 orange |
+| ≤ 25% | ok |
+
+---
+
+### PEP — Parallel Execution Potential
+
+```
+PEP = active workstreams with all deps completed / (active + blocked)
+```
+
+Estimates how much work can proceed right now. A workstream is eligible if its status is `active` and every workstream it depends on has reached `completed` or `archived`.
+
+| PEP | Warning |
+|---|---|
+| < 30% | 🔴 red |
+| 30–60% | 🟠 orange |
+| ≥ 60% | ok |
+
+---
+
+### CDDR — Cross-Domain Dependency Ratio
+
+```
+CDDR = dependency edges crossing domain boundaries / total edges
+```
+
+Measures architectural entanglement. High CDDR indicates loss of modularity across the six project domains.
+
+| CDDR | Warning |
+|---|---|
+| > 40% | 🟠 orange |
+
+---
+
+### CPI — Cycle Presence Indicator
+
+```
+CPI = 0  →  no cycles
+CPI = 1  →  at least one circular dependency detected
+```
+
+Detected via DFS with inStack colouring. Any cycle means no feasible execution order exists — a structural deadlock. When CPI = 1, the final WHI score is **halved** as a hard penalty.
+
+---
+
+## Aggregation formula
+
+```
+DDnorm = min(1, DD / 1.0)          ← saturates at DD_critical = 1.0
+
+WHI = 0.30 × (1 − DDnorm)
+    + 0.25 × (1 − BR)
+    + 0.15 × (1 − SPR)
+    + 0.20 × PEP
+    + 0.10 × (1 − CDDR)
+
+if CPI = 1:  WHI = WHI × 0.5
+```
+
+Result is clamped to [0, 1].
+
+---
+
+## Domain breakdown
+
+The card also shows a per-domain WHI computed using **intra-domain workstreams and intra-domain edges only**. This measures each domain's internal autonomy — how well its workstreams are decomposed relative to each other, independent of cross-domain dependencies.
+
+A domain with WHI = 100% is fully self-contained and parallelizable internally. Its global contribution to the program-level WHI may still be reduced by cross-domain dependencies (captured in CDDR).
+
+The domain breakdown is shown when at least two domains have active workstreams.
+
+---
+
+## How to improve a poor score
+
+| Symptom | Action |
+|---|---|
+| High DD | Decompose tightly coupled workstreams; remove unnecessary dependencies |
+| High BR | Unblock workstreams — resolve the blocking condition, or mark dependency as completed if done |
+| High SPR | Split the bottleneck workstream into independent deliverables |
+| Low PEP | Complete prerequisite workstreams or re-sequence work |
+| High CDDR | Refactor cross-domain dependencies into shared contracts or invert the dependency |
+| CPI = 1 | Find and break the cycle — identify which dependency edge is incorrect and remove it |
+
+---
+
+## What WHI is not
+
+WHI measures **structural health of the work graph** — not individual performance, not velocity, not burn-down rate. A team can be moving fast with a poor WHI (serially but quickly), or slowly with a perfect WHI (fully parallel but under-resourced). Use WHI alongside velocity metrics, not instead of them.
+
+---
+
+*Specification: `state-hub/dashboard/src/docs/workstream-kpi.md`*