Add selected single-file persistent tree store

store-tree.c keeps every document in one append-only file as a persistent
B-tree of byte extents, with the key catalog in the same structure. A splice
writes only the changed leaf and path; unchanged extents are shared between
versions, readers take no lock, and two alternating head slots publish a
commit after its nodes are durable. Compaction rewrites reachable nodes.

On the recorded ext4 host a durable 1-byte middle insert into a 1 MiB
document took 8.58 ms against 78.51 ms for the default store, and the real
TUI's Ctrl+S after typing took 16.60 ms against 84.59 ms. check-tree runs the
store's crash/oracle tests and the unchanged runtime, splice, async and
real-PTY editor checks on it. It stays opt-in: linked code is 12.7 KB larger
and contiguous multi-leaf reads copy.

1 files changed, 346 insertions, 0 deletions

diff --git a/research/experiments/008-tree-store-edit-cost/2026-09-25-ext4-linux-aarch64.json b/research/experiments/008-tree-store-edit-cost/2026-09-25-ext4-linux-aarch64.json
new file mode 100644
index 0000000..708346f
--- /dev/null
+++ b/research/experiments/008-tree-store-edit-cost/2026-09-25-ext4-linux-aarch64.json
@@ -0,0 +1,346 @@
+{
+  "device": "/dev/mmcblk0p2",
+  "filesystem": "ext4",
+  "machine": "aarch64",
+  "kernel": "6.18.39+rpt-rpi-2712",
+  "compiler": "cc (Debian 14.2.0-19) 14.2.0",
+  "edits_per_scenario": 32,
+  "results": [
+    {
+      "store": "flat",
+      "op": "insert-middle",
+      "document_bytes": 256,
+      "edits": 32,
+      "edit_median_ms": 9.305,
+      "edit_p90_ms": 20.037,
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+      "write_call_bytes_per_edit": 296,
+      "device_write_bytes_per_edit": 16384,
+      "store_file_bytes_after": 312,
+      "whole_read_median_ms": 0.008,
+      "read_checksum": 7776
+    },
+    {
+      "store": "paged",
+      "op": "insert-middle",
+      "document_bytes": 256,
+      "edits": 32,
+      "edit_median_ms": 8.463,
+      "edit_p90_ms": 11.859,
+      "edit_max_ms": 64.024,
+      "write_call_bytes_per_edit": 296,
+      "device_write_bytes_per_edit": 16384,
+      "store_file_bytes_after": 312,
+      "whole_read_median_ms": 0.01,
+      "read_checksum": 7776
+    },
+    {
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+      "op": "insert-middle",
+      "document_bytes": 256,
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+      "whole_read_median_ms": 0.011,
+      "read_checksum": 7776
+    },
+    {
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+    },
+    {
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+    },
+    {
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+    {
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+  ]
+}