fix(process): localize capture truncation diagnostics (#1284)

Report per-source capture coverage and existing file-hash omissions while preserving comparable observations in unaffected dimensions. Keep legacy truncated captures conservative and retain known partial trace events.

Closes #1250
Partially addresses #1251; independent hashing remains out of scope.

src/process/capture/ProcessSampling.ts

@@ -1,4 +1,5 @@
 import { readFile, readdir } from "node:fs/promises";
+import type { ProcessSamplingCoverage } from "../../domain/process/processCaptureCoverage.js";
 import type {
   ProcessSample,
   RecordProcessCaptureEvent,
@@ -455,7 +456,10 @@ export const startProcessSampler = (options: {
   readonly limit: number;
   readonly samples: ProcessSample[];
   readonly recordEvent?: RecordProcessCaptureEvent;
-}): (() => Promise<{ readonly partial: boolean }>) => {
+}): (() => Promise<{
+  readonly partial: boolean;
+  readonly coverage: ProcessSamplingCoverage;
+}>) => {
   const { rootPid, runId, started, limit, samples } = options;
   const identities = new Map<number, string>();
   const lastObservations = new Map<number, string>();
@@ -463,6 +467,9 @@ export const startProcessSampler = (options: {
   let stopped = false;
   let abortCurrent: (() => void) | undefined;
   let partial = false;
+  let limitReached = false;
+  let samplingFailures = 0;
+  let firstSamplingFailure: string | null = null;
   let rootInitialized = false;
 
   const sample = (): void => {
@@ -501,6 +508,7 @@ export const startProcessSampler = (options: {
           if (lastObservations.get(value.pid) === observation) continue;
           if (samples.length >= limit) {
             partial = true;
+            limitReached = true;
             continue;
           }
           lastObservations.set(value.pid, observation);
@@ -510,8 +518,16 @@ export const startProcessSampler = (options: {
         }
       })
       .catch((cause: unknown) => {
-        if (!(cause instanceof Error && cause.name === "AbortError"))
+        if (!(cause instanceof Error && cause.name === "AbortError")) {
           partial = true;
+          samplingFailures += 1;
+          firstSamplingFailure ??=
+            cause instanceof Error
+              ? `${cause.name}: ${cause.message}`
+              : typeof cause === "string"
+                ? cause
+                : `Unexpected sampling rejection (${typeof cause})`;
+        }
       })
       .then(() => {
         abortCurrent = undefined;
@@ -527,6 +543,17 @@ export const startProcessSampler = (options: {
     clearInterval(timer);
     abortCurrent?.();
     await pending;
-    return { partial };
+    return {
+      partial,
+      coverage: {
+        sample_limit: limit,
+        sampling_partial: partial,
+        retained_samples: samples.length,
+        sample_limit_reached: limitReached,
+        sampling_failures: samplingFailures,
+        first_sampling_failure: firstSamplingFailure,
+        coverage: "sampled",
+      },
+    };
   };
 };