Reference-chains design and architecture docs

[jbachorik]

What does this PR do?:
Adds the reference-chains documentation: the design document, the as-built implementation reference, the signals explainer (search gate, pain budgets, urgency, leak tags), and the collection summary.

Motivation:
Documentation layer for PROF-15341, matching the as-built state of the engine shipped earlier in the stack.

Additional Notes:
Stacked on #801. Docs only — no code.

How to test the change?:
N/A — documentation only.

For Datadog employees:

• This PR doesn't touch any of that.
• JIRA: PROF-15341

[dd-octo-sts]

Scan-Build Report

User:	runner@runnervmlun5p
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 all
Clang Version:	Ubuntu clang version 18.1.3 (1ubuntu1)
Date:	Thu Sep 17 20:07:38 2026

Bug Summary

Bug Type	Quantity	Display?
All Bugs	4
C++ move semantics
Use-after-move	1
Logic error
Dereference of null pointer	1
Result of operation is garbage or undefined	1
Unused code
Dead increment	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Unused code	Dead increment	referenceChains.cpp	collectStaticFieldAnchorsForRotation	3677	1
Logic error	Dereference of null pointer	faultInjection.cpp	crashNow	24	2
Logic error	Result of operation is garbage or undefined	livenessTracker.cpp	secondsToOOM	1383	24
C++ move semantics	Use-after-move	referenceChains.cpp	buildCanaryChainEvent	6083	68

[dd-octo-sts]

CI Test Results

Run: #35577106729 | Commit: 8ff7e37 | Duration: 1h 29m 34s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

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Updated: 2026-09-21 09:50:59 UTC

[dd-octo-sts]

✅ All 40 integration tests passed

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