Preserve Tier2 selector-byte source evidence

docs/rehost-queue/tier2-selector-byte-sources-2026-04-21.md

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+# Tier2 Selector-Byte Sources (2026-04-21)
+
+This note preserves the stock selector-byte evidence behind the current Tier-2 queue head, so the
+short queue does not have to restate the `BuildingTypes` side every time.
+
+## Checked Export
+
+- `artifacts/exports/rt3-1.06/building-type-sources.json`
+
+## Stock `.bca` Selector Bytes
+
+The checked export keeps the raw `0xb8..0xbb` selector-byte window explicit for every `.bca` file:
+
+- `bca_file_count = 77`
+- `bca_selector_pattern_count = 8`
+- only one checked `.bca` pattern is nonzero at all:
+  - `MachineShop.bca`
+  - `byte_0xb8 = 0x00`
+  - `byte_0xb9 = 0x80`
+  - `byte_0xba = 0x3f`
+  - `byte_0xbb = 0x00`
+
+Everything else in the shipped `.bca` selector-byte corpus stays zero at `0xb8..0xbb`.
+
+## Nonzero `.bty` Alias Family
+
+The checked recovered-table summaries keep the nonzero `.bty` alias family explicit too:
+
+- `nonzero_bty_header_alias_selector_summaries`
+  - `17` grouped selector summaries
+  - `22` files total
+- the strongest same-root family is the `TextileMill` alias-root group:
+  - `dword_0xbb = 0x000001f4`
+  - `byte_0xba = 0x00`
+  - `byte_0xbb = 0x00`
+  - `9` files
+  - sample files include `Port.bty`, `Warehouse.bty`, `ConcretePlant.bty`,
+    `ConstructionFirm.bty`, and `ElectronicsPlant.bty`
+
+The next smaller nonzero alias-root groups are:
+
+- `LumberMill`
+  - `4` files
+- `MeatPackingPlant`
+  - `4` files
+- `Distillery`
+  - `2` files
+- `Toolndie`
+  - `2` files
+
+## Immediate Reading
+
+This keeps the Tier-2 source side narrower than a generic “stock assets might hide selector bytes”
+claim:
+
+- the shipped `.bca` selector-byte corpus is almost entirely zero
+- the one checked nonzero `.bca` outlier is `MachineShop.bca`
+- the broader stock-side nonzero signal sits instead in the `.bty` alias-root family with
+  `dword_0xbb = 0x000001f4`, especially the `TextileMill` branch that already covers `Port.bty`
+  and `Warehouse.bty`
+
+So the remaining Tier-2 question is still not “do the stock source files carry any selector-byte
+signal at all?” It is which earlier seed or projection seam lifts that narrow stock-side signal
+into the live candidate bank bytes `[candidate+0xba/+0xbb]` before `0x00412d70` and
+`0x00419230` consume it.