docs(changelog): fragment for #8000 (GC-strategy define-header pin)

Author: Ralph Küpper

changelog.d/8000-gc-strategy-define-header-pin.md

@@ -0,0 +1,30 @@
+### Codegen: pin the GC strategy onto the define line, in both root lowerings (#7982 follow-up)
+
+#7998 made `LlFunction::define_header` the single renderer of the `define … {`
+line, after the in-process native path's private copy silently lost
+`gc "statepoint-example"` — a natively-constructed module then got no RS4GC
+pass and therefore **no precise roots at all**, while verifying, linking and
+running correctly on any program that does not collect.
+
+The test that shipped with it does not actually pin that property, and the two
+attempts to fix it failed the same way the original bug did. Recorded because
+it is this change's own bug class occurring inside its own test:
+
+1. The `to_ir` == `define_header` agreement test **cannot** see a dropped
+   strategy: with one shared renderer both sides change identically. Sabotage
+   passed.
+2. A dedicated strategy test that branched on `native_stack_roots_enabled()`
+   never ran its ON arm under `cargo test` — no module has called
+   `set_native_roots_for_target`, so the predicate is false in the test
+   process. Sabotage passed again.
+3. Only pinning **both** lowerings with `NativeRootsPin::{native,shadow}`, and
+   asserting `stack_map_slot_count` in each arm first so neither can pass
+   vacuously, goes red on the sabotage.
+
+Native-roots must take the stack-map path AND name the strategy; shadow-stack
+must not take it AND must not name the strategy. The tests live in
+`function.rs`, which compiles WITHOUT the `llvm-inprocess` feature, so they run
+in per-PR `cargo-test` rather than only in the feature job.
+
+Sabotage-verified with the fix committed first, then restored and REBUILT: 935
+`perry-codegen` lib tests green under `--features llvm-inprocess`.