bi-01 step 03 — Archaeology and layering

Goal

Use history and build metadata to answer why, not just what — the two questions source alone cannot answer.

Date a behaviour. Use git log -S'<exact string>' to find the commit that introduced a special case you do not understand. Read its message and its Bug: footer.
```
git log -S'CausesFosterParenting' --oneline -- third_party/blink/renderer/core/html/parser/
```
Date a requirement. Find the commit that raised Chromium's macOS SDK floor:
```
git log -1 --format='%h %ad %s' --date=short \
  -S'mac_sdk_official_version = "26.5"' -- build/config/mac/mac_sdk.gni
```
A build failure with a date attached is a decision; without one it is a mystery.
Survive a rename. Use git log --follow on a layout file to see it across the ng_* rename. Confirm for yourself that any source using ng_ prefixes is stale.
Read the layering. Open third_party/blink/renderer/DEPS and one core/*/DEPS. Find one include rule you would not have predicted, and one documented exception with a justifying comment.

Read the build graph. For a file you have studied:

gn refs out/Default <file>          # which targets contain it
gn path out/Default //A //B         # why does A depend on B

(If the build is blocked, do steps 1–4; they need only the checkout.)