test(memory): add direct unit tests for FlowMemoryHandler

Cover the memory feature blind spot (line coverage 23.17 % → 97.59 %).
The handler was previously exercised only indirectly through
FlowExecutor parallel paths, leaving setup/tick/shouldKill/enrichResults
/attachStats unprotected against regressions.

- src/Execution/FlowMemoryHandler.php: drop `final` and add two
  protected seams — `buildSampler()` (factory escape hatch) and
  `emitWarning()` (degradation-warning escape hatch). Production
  behavior is unchanged: subclasses are an opt-in test mechanism.
- tests/Doubles/FakeMemorySampler.php: configurable MemorySampler test
  double that records every sample() call and replays a per-call RSS
  sequence.
- tests/Unit/Execution/FlowMemoryHandlerTest.php: 24 tests covering
  the seven activation paths in setup() (no triggers, job threshold,
  job reserve, flow budget, --stats, disabled flag, sampler-unavailable
  warning), tick() PID propagation and null-pid filtering, shouldKill()
  on every branch (disabled, inactive, peak ≥ fail-above, peak < fail-
  above), enrichResults() peak/reserve/threshold attachment with the
  disabled-flag and orphan-result branches, and attachStats() budget-
  state and stats-block attachment.

Author: Test

src/Execution/FlowMemoryHandler.php

@@ -23,7 +23,7 @@
  *
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects) Touches every piece of the memory feature.
  */
-final class FlowMemoryHandler
+class FlowMemoryHandler
 {
     private OptionsConfiguration $options;
 
@@ -79,11 +79,10 @@ public function setup(array $jobs): bool
             return false;
         }
 
-        $sampler = (new MemorySamplerFactory())->create();
+        $sampler = $this->buildSampler();
         if (!$sampler->isAvailable() && $thresholdsRequested) {
-            fwrite(
-                STDERR,
-                '⚠ Memory budget disabled: ' . $sampler->getUnavailableReason() . PHP_EOL
+            $this->emitWarning(
+                '⚠ Memory budget disabled: ' . $sampler->getUnavailableReason()
             );
         }
 
@@ -95,6 +94,26 @@ public function setup(array $jobs): bool
         return true;
     }
 
+    /**
+     * Seam for tests: production builds a real sampler via the factory; tests
+     * subclass and inject a fake to exercise the handler without touching
+     * /proc or shelling out to `ps`.
+     */
+    protected function buildSampler(): MemorySampler
+    {
+        return (new MemorySamplerFactory())->create();
+    }
+
+    /**
+     * Seam for tests: production writes to STDERR; tests override to capture
+     * the one-time degradation warning emitted when the sampler is
+     * unavailable but thresholds were declared (REQ-038).
+     */
+    protected function emitWarning(string $message): void
+    {
+        fwrite(STDERR, $message . PHP_EOL);
+    }
+
     public function isActive(): bool
     {
         return $this->evaluator !== null;

tests/Doubles/FakeMemorySampler.php

@@ -0,0 +1,57 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Doubles;
+
+use Wtyd\GitHooks\Execution\Memory\MemorySampler;
+
+/**
+ * Test double for MemorySampler: returns a configurable per-job RSS map
+ * on every sample() call and exposes the call history so tests can assert
+ * which PIDs were sampled at each tick.
+ */
+final class FakeMemorySampler implements MemorySampler
+{
+    /** @var array<int, array<string, int>> Sequence of RSS maps to return per sample() call. */
+    private array $rssSequence;
+
+    private bool $available;
+
+    private string $unavailableReason;
+
+    /** @var array<int, array<string, int>> Recorded jobNameToPid arguments per call. */
+    public array $calls = [];
+
+    /**
+     * @param array<int, array<string, int>> $rssSequence
+     *   Per-call sequence of jobName→RSS maps. When more sample() calls happen
+     *   than entries provided, the last entry is reused.
+     */
+    public function __construct(
+        array $rssSequence = [[]],
+        bool $available = true,
+        string $unavailableReason = ''
+    ) {
+        $this->rssSequence = $rssSequence === [] ? [[]] : $rssSequence;
+        $this->available = $available;
+        $this->unavailableReason = $unavailableReason;
+    }
+
+    public function sample(array $jobNameToPid): array
+    {
+        $this->calls[] = $jobNameToPid;
+        $idx = min(count($this->calls) - 1, count($this->rssSequence) - 1);
+        return $this->rssSequence[$idx];
+    }
+
+    public function isAvailable(): bool
+    {
+        return $this->available;
+    }
+
+    public function getUnavailableReason(): string
+    {
+        return $this->unavailableReason;
+    }
+}