refactor(runtime): split field_set_by_name.rs (#7402)

changelog.d/7402-split-field-set-by-name.md

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+Split `crates/perry-runtime/src/object/field_set_by_name.rs` into topical
+sub-modules (issue #7402). The file sat at exactly 2000 of the 2000 lines
+`scripts/check_file_size.sh` allows and is not allowlisted, so the next line
+added anywhere in it turned the required `lint` context red — and a red
+required context means every subsequent merge bypasses a gate rather than
+being blocked by one. Trimming comments back to exactly 2000 (as was done
+earlier) is a fuse, not a fix.
+
+Pure mechanical relocation — no behaviour change. Every moved statement is
+byte-identical to its `origin/main` text; the only edits are four
+file-private `unsafe fn`s becoming `pub(super)` so the new sibling modules
+can reach them, and one now-dead `#[allow(unused_assignments)]` dropped from
+the entry point (the assignments it covered moved out with the macro).
+
+    field_set_by_name.rs          568  entry point + pre-rooting head
+    field_set_by_name/tail.rs     963  rooted ObjectHeader write walk
+    field_set_by_name/fast_paths.rs  284  non-rooting fast paths
+    field_set_by_name/write_helpers.rs  170  key/diag utils, closure +
+                                             class-mirror + namespace writes
+    field_set_by_name/attr_variants.rs   87  nonenum / nonconfigurable
+
+The tail's split point is the `RuntimeHandleScope` creation, chosen so the
+`refresh_roots_after_alloc!()` macro (#7341) and all 16 of its call sites —
+plus both `GC_STORE_AUDIT(INIT)` markers and the #7154/#6759 publication
+ordering they document — stay together in one file with their order
+untouched. No rooted step crosses a module boundary.
+
+`scripts/addr_class_allowlist.txt` and `scripts/addr_class_ratchet_baseline.txt`
+are re-keyed for the new paths: both are path-prefix/per-file-count keyed, so
+a file split redistributes existing sites across new paths and would
+otherwise read as a ratchet regression. No site was added or removed.