feat: chained signal composition — 3→1→final + mood mixing

Add compose-metrics.ds (Layer 1): receives counter+clock+stats,
derives uptime/events/status, re-streams on /peer/metrics. This app
is BOTH a receiver and a source.

Add compose-master.ds (Layer 2): receives chained metrics from
Layer 1 + mood direct from Layer 0. Demonstrates multi-layer
signal composition with independent stream mixing.

Verified: Uptime: 51s, Total Events: 9 flowing through the full
three-layer chain to the master dashboard.

examples/compose-master.ds

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+-- DreamStack Chained Composition — Layer 2: Master Dashboard
+-- Receives the aggregated metrics (from Layer 1) AND mood (from Layer 0).
+--
+-- Architecture:
+--   Counter ──┐
+--   Clock  ───┤──► compose-metrics ──┐
+--   Stats  ───┘    (Layer 1)         │
+--                                    ├──► THIS APP (Layer 2)
+--   Mood ────────────────────────────┘
+--
+-- This demonstrates:
+--   1. Chained composition (3 → 1 → final)
+--   2. Multiple layers of signal derivation
+--   3. Mixing independently-sourced streams
+--
+-- Run with:
+--   dreamstack build examples/compose-master.ds
+
+let metrics = stream from "ws://localhost:9100/stream/metrics"
+let mood = stream from "ws://localhost:9100/stream/mood"
+
+view main =
+  column [
+    text "🏗️ Master Dashboard (Layer 2)"
+    text "Chained: 3→1→here + mood"
+    row [
+      column [
+        text "── ⚡ Aggregated Metrics ──"
+        text "(from compose-metrics)"
+        text "Uptime: {metrics.uptime}s"
+        text "Total Events: {metrics.events}"
+        text "Status: {metrics.status}"
+      ]
+      column [
+        text "── 😊 Mood (Direct) ──"
+        text "(from streaming-mood)"
+        text "Mood: {mood.mood}"
+        text "Energy: {mood.energy}%"
+        text "Color: {mood.color}"
+      ]
+    ]
+  ]

examples/compose-metrics.ds

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+-- DreamStack Chained Composition — Layer 1: Metrics Aggregator
+-- Receives 3 signal streams, derives combined metrics, re-streams them.
+--
+-- Architecture:
+--   Counter ──┐
+--   Clock  ───┤──► THIS APP ──► /peer/metrics
+--   Stats  ───┘
+--
+-- This app is BOTH a receiver AND a source:
+--   - Receives: counter.count, counter.doubled, clock.seconds, stats.total, stats.max
+--   - Derives:  uptime (from clock), events (from stats+counter), status (combined)
+--   - Streams:  uptime, events, status on /peer/metrics
+--
+-- Run with:
+--   dreamstack stream examples/compose-metrics.ds --port 3006
+
+let counter = stream from "ws://localhost:9100/stream/counter"
+let clock = stream from "ws://localhost:9100/stream/clock"
+let stats = stream from "ws://localhost:9100/stream/stats"
+
+-- Derived metrics from upstream streams
+let uptime = clock.seconds
+let events = counter.count + stats.total
+let status = "nominal"
+
+-- Re-stream derived metrics downstream
+stream metrics on "ws://localhost:9100/peer/metrics" {
+  mode: signal,
+  output: uptime, events, status
+}
+
+view metrics =
+  column [
+    text "⚡ Metrics Aggregator (Layer 1)"
+    text "Receives: counter + clock + stats"
+    text "Uptime: {uptime}s"
+    text "Total Events: {events}"
+    text "Status: {status}"
+  ]