test(memtrack): reproduce fork-inherited unmap removes-exceed-adds

[not-matthias]

The rmap resident estimate in the parser is a signed running sum per pid
starting at zero. Fork copies page tables through the folio_*_dup_*
helpers, which the rmap program does not hook, so pages inherited at
fork never produce adds under the child's pid. Unmapping or reclaiming
them fires in-context removes with no matching adds, leaving a
net-negative delta — the root cause behind the negative resident
estimates the parser now floors at zero.

This adds a deterministic, CI-safe repro (no memory pressure needed):
the fixture mmaps and touches 32 MiB, forks a child that munmaps the
inherited region and exits. The test asserts the child's anon rmap net
delta is at least ~-24 MiB with no adds, documenting the event-stream
contract that consumers must tolerate removes exceeding adds per pid.

Companion to the parser-side fix in CodSpeedHQ/platform#2729.

[greptile-apps]

RSS test infrastructure is absent

When Cargo compiles the new integration-test target, its compile-time fixture includes, shared::compile_c_source and shared::track_command_with_rmap calls, and RSS event variants are unavailable in the reviewed tree, causing cargo test to fail before the tests can run.

Knowledge Base Used: memtrack: eBPF-based memory allocation tracking

Prompt To Fix With AI

This is a comment left during a code review.
Path: crates/memtrack/tests/rss_tests.rs
Line: 184-186

Comment:
**RSS test infrastructure is absent**

When Cargo compiles the new integration-test target, its compile-time fixture includes, `shared::compile_c_source` and `shared::track_command_with_rmap` calls, and RSS event variants are unavailable in the reviewed tree, causing `cargo test` to fail before the tests can run.

**Knowledge Base Used:** [memtrack: eBPF-based memory allocation tracking](https://app.greptile.com/codspeed/-/custom-context/knowledge-base/codspeedhq/codspeed/-/docs/memtrack.md)

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

[greptile-apps]

Greptile Summary

Adds a deterministic fork-inherited unmap reproduction and an RSS/rmap integration-test target.

• Allocates and faults 32 MiB before forking, then unmaps the inherited mapping in the child.
• Asserts that the child's anonymous rmap delta is net-negative by approximately the inherited region size.
• Introduces broader RSS/rmap aggregation and regression-test helpers.

Confidence Score: 2/5

This PR is not safe to merge until the new RSS integration-test target's missing fixtures and supporting APIs are included or the target is reduced to available infrastructure.

Cargo test discovers the new integration-test file and encounters compile-time includes, helper calls, and event variants that do not exist in the reviewed tree, preventing the test target from compiling.

Files Needing Attention: crates/memtrack/tests/rss_tests.rs

Important Files Changed

Filename	Overview
crates/memtrack/testdata/rss/fork_unmap_inherited.c	Adds the focused C fixture that faults a private anonymous mapping and unmaps its inherited copy in a child.
crates/memtrack/tests/rss_tests.rs	Adds the intended regression assertion alongside a larger test suite, but the new target cannot compile without missing fixtures, helpers, and event variants.

Sequence Diagram

sequenceDiagram
    participant Test as rss_tests
    participant Parent as Fixture parent
    participant Child as Forked child
    participant Rmap as rmap event stream
    Test->>Parent: mmap and touch 32 MiB
    Parent->>Child: fork with inherited page tables
    Note over Rmap,Child: No child add events for inherited pages
    Child->>Rmap: munmap emits child remove events
    Test->>Rmap: Sum child anonymous deltas
    Rmap-->>Test: "Net delta <= -24 MiB"

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Prompt To Fix All With AI

### Issue 1
crates/memtrack/tests/rss_tests.rs:184-186
**RSS test infrastructure is absent**

When Cargo compiles the new integration-test target, its compile-time fixture includes, `shared::compile_c_source` and `shared::track_command_with_rmap` calls, and RSS event variants are unavailable in the reviewed tree, causing `cargo test` to fail before the tests can run.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "test(memtrack): reproduce fork-inherited..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 17 untouched benchmarks

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_{Comparing test/memtrack-fork-inherited-unmap-contract (934749c) with main (0418479)}