Add the shared pull-request template (taphouse sync)

.github/pull_request_template.md

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+## What this changes
+
+<!-- One or two sentences. Link the issue if there is one. -->
+
+## Why
+
+<!-- The problem this solves, not a restatement of the diff. -->
+
+## Verification
+
+<!-- What you actually built and ran, and what you did not — state the gap
+     plainly rather than leaving it to be inferred. If CI is the first real
+     gate for part of this change, say so.
+
+     Every performance claim cites the executed notebook cell or the pinned
+     test that carries the number: measured, not remembered. -->
+
+## Notes for the reviewer
+
+<!-- Delete the lines that do not apply; add anything that surprised you.
+
+  - **Contract change.** A documented contract point moved — packing,
+    conventions, normalization, latency, a Q format, a default. This is
+    breaking for every consumer; say which ones.
+  - **Submodule pin moved.** Which pin, and why. After this merges by
+    rebase/squash, repoint any open consumer pins at the identical tree on
+    `main` so they stay reachable once the branch is deleted.
+  - **Notebooks re-executed**, because behavior changed. They are committed
+    executed.
+  - **Style configs** came from a `taphouse` sync, not a hand-edit. A
+    hand-edited copy fails the drift job by design.
+  - **Canonical TapHouse file changed.** Tag it, then re-sync consumers —
+    merge here first, since a guarded unconditional file must exist at the
+    pinned ref before a consumer references it.
+  - **Max/Pd package.** Reference page and help patcher ship for each new
+    object; the macOS binary is still universal (arm64 + x86_64).
+  - **Documented exception.** This repo diverges from a house rule on
+    purpose (see its `.clang-tidy` / `STYLE.md` note) and this change keeps
+    that divergence intact.
+-->