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, 285 insertions, 0 deletions

diff --git a/research/experiments/008-tree-store-edit-cost/2026-09-25-tui-ext4-linux-aarch64.json b/research/experiments/008-tree-store-edit-cost/2026-09-25-tui-ext4-linux-aarch64.json
new file mode 100644
index 0000000..6bea961
--- /dev/null
+++ b/research/experiments/008-tree-store-edit-cost/2026-09-25-tui-ext4-linux-aarch64.json
@@ -0,0 +1,285 @@
+{
+  "platform": "Linux-6.18.39+rpt-rpi-2712-aarch64-with-glibc2.41",
+  "filesystem": "/dev/mmcblk0p2 ext4",
+  "document_bytes": 1048576,
+  "rounds": 3,
+  "edits_per_session": 8,
+  "median_ms": {
+    "replace": {
+      "flat": 86.32,
+      "paged": 21.31,
+      "tree": 16.97
+    },
+    "append": {
+      "flat": 84.59,
+      "paged": 93.29,
+      "tree": 16.6
+    }
+  },
+  "binaries": {
+    "flat": {
+      "cli_sha256": "f69c7580104b873e0736d7503dcbbcbb6b180adf54920e6ba741d363262867b8",
+      "tui_sha256": "644980089e65e9db01ecdd21c4edf8a0ae7fe248114340b3b691e6c283e5e5b4"
+    },
+    "paged": {
+      "cli_sha256": "233311ee133bc1d5299963f4523131dac542d71013cb825f8a4eaa48de813ee8",
+      "tui_sha256": "77169c79a800881e901e5b1bba938543eed5f3f1f0914799e56c0635859af2d1"
+    },
+    "tree": {
+      "cli_sha256": "c0c783a1b30a1f8e88bfe9a09b6ae124f596bf6239b802e4aec5e56f374714b9",
+      "tui_sha256": "8030475c663b09791a5fa24bad2f8498f652d4361a6734a737b3c6a22e4659aa"
+    }
+  },
+  "runs": {
+    "replace": {
+      "flat": [
+        {
+          "samples_ns": [
+            83883854,
+            32709073,
+            121001564,
+            92289945,
+            96340473,
+            90943269,
+            47708564,
+            89206225
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        },
+        {
+          "samples_ns": [
+            85403197,
+            33045701,
+            71709920,
+            88249419,
+            87284740,
+            30588422,
+            86124025,
+            86515579
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        },
+        {
+          "samples_ns": [
+            85907991,
+            52699698,
+            36407605,
+            91580804,
+            91115363,
+            96859361,
+            91057308,
+            30358535
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        }
+      ],
+      "paged": [
+        {
+          "samples_ns": [
+            21286170,
+            21523669,
+            20435102,
+            21175041,
+            24219650,
+            19863957,
+            21457280,
+            20223807
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        },
+        {
+          "samples_ns": [
+            20820044,
+            18812854,
+            22029072,
+            19884550,
+            21458169,
+            21256596,
+            25127069,
+            73009317
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        },
+        {
+          "samples_ns": [
+            21277781,
+            23763283,
+            18573541,
+            21452318,
+            21336634,
+            75828726,
+            18893984,
+            21467613
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        }
+      ],
+      "tree": [
+        {
+          "samples_ns": [
+            16691108,
+            17111328,
+            15930836,
+            17254197,
+            16401425,
+            16596591,
+            17847323,
+            18489726
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        },
+        {
+          "samples_ns": [
+            16340445,
+            18211228,
+            12615914,
+            17297716,
+            17568492,
+            16601387,
+            17259642,
+            15886466
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        },
+        {
+          "samples_ns": [
+            17321808,
+            16796238,
+            16831978,
+            15132138,
+            17392253,
+            12584915,
+            17342679,
+            17423900
+          ],
+          "body_sha256": "1acbea9ad5e6321b7e4f8b3f85450d99cb2d82b655820160dcd18bcaae58e00f"
+        }
+      ]
+    },
+    "append": {
+      "flat": [
+        {
+          "samples_ns": [
+            85268681,
+            58706620,
+            80969562,
+            30847551,
+            88231013,
+            85485051,
+            29563059,
+            88786509
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        },
+        {
+          "samples_ns": [
+            58646271,
+            30913440,
+            89689432,
+            85278758,
+            33291979,
+            86740285,
+            83913915,
+            90844368
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        },
+        {
+          "samples_ns": [
+            51920946,
+            28831473,
+            85487720,
+            85906421,
+            85835644,
+            32848520,
+            85315555,
+            82713850
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        }
+      ],
+      "paged": [
+        {
+          "samples_ns": [
+            91333586,
+            37920058,
+            92091377,
+            63703624,
+            95308984,
+            40485355,
+            99081866,
+            93075370
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        },
+        {
+          "samples_ns": [
+            96352460,
+            93512017,
+            65222596,
+            38763552,
+            98852258,
+            131612999,
+            138169789,
+            131450371
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        },
+        {
+          "samples_ns": [
+            40996002,
+            93012356,
+            63056206,
+            93559778,
+            37065306,
+            98450597,
+            112059339,
+            111368899
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        }
+      ],
+      "tree": [
+        {
+          "samples_ns": [
+            15267508,
+            17113014,
+            17454104,
+            16570055,
+            69532029,
+            15171139,
+            18091600,
+            11101536
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        },
+        {
+          "samples_ns": [
+            19964902,
+            14639290,
+            16407186,
+            17425161,
+            15285638,
+            18012231,
+            15061398,
+            17329272
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        },
+        {
+          "samples_ns": [
+            17466771,
+            16352390,
+            16629406,
+            15132064,
+            18861503,
+            11662162,
+            16306335,
+            17256958
+          ],
+          "body_sha256": "bd42946a8dafa764907691fbebfd981ef4b012fbcbf5c25beab7d6b1c8d0cc30"
+        }
+      ]
+    }
+  }
+}