Establish native deterministic codegen framework (#45)

codegen/README.md

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+# Native data generation
+
+`libremetaverse-codegen` is the Rust-only framework for the six source
+generators present at LibreMetaverse commit
+`2aa70bb68513b39795da5d13c88f31b86e85a3ba`. Normal builds compile only the
+checked-in Rust outputs. They do not run this tool, invoke .NET or Roslyn, read
+an environment-dependent path, access the network, or open the sibling
+reference checkout.
+
+## Provenance inventory
+
+[`sources.json`](sources.json) is the machine-readable inventory. It records
+the upstream repository and commit, source-generator hashes, every actual
+`AdditionalFile`, its vendored path, SHA-256, format, and BSD-3-Clause license.
+
+| Golden generator | Pinned data inputs |
+| --- | --- |
+| PacketSourceGenerator | `message_template.msg` |
+| VisualParamGenerator | `avatar_lad.xml` |
+| SkeletonGenerator | `avatar_skeleton.xml` |
+| AttentionsGenerator | `attentions.xml`, `attentionsN.xml` |
+| TreesGenerator | `trees.xml`, `grass.xml` |
+| GenepoolGenerator | `genepool.xml` |
+
+The C# project does not provide the optional `visualparamtemplate.cs` file.
+The pinned VisualParam generator therefore uses the template embedded in its
+own source; the inventory records that generator source and hash as provenance.
+
+The files under `inputs/` are exact, unmodified snapshots. Their license and
+copyright attribution are covered by the repository
+[`LICENSE.md`](../LICENSE.md). The generated source manifest repeats each hash
+and license in its header and data table.
+
+## Commands
+
+Regenerate every currently registered Rust output using only checked-in data:
+
+```console
+cargo run -p libremetaverse-codegen -- generate
+```
+
+Verify hashes and reject stale output without writing:
+
+```console
+cargo run -p libremetaverse-codegen -- check
+```
+
+Maintainers can refresh the vendored snapshots from an explicitly supplied
+checkout. The command verifies all six golden generator hashes and all input
+hashes before writing, so a different upstream revision is rejected:
+
+```console
+cargo run -p libremetaverse-codegen -- vendor ../libremetaverse
+```
+
+Generation uses UTF-8 with an optional BOM, normalizes CRLF/CR to LF, rejects
+NUL input, reports stable `path:line:column: severity[code]` diagnostics, sorts
+manifest records by stable IDs, emits provenance-rich generated headers, and
+always terminates output with one LF. Tests generate twice in memory and
+require byte identity before comparing with the checked-in file.