chore: add repo hygiene config and project spec

Bootstrap the repository with the shared configuration every other piece
depends on:

- .gitignore: strict. Keeps build output, cargo caches, Android keystores,
  signing keys, .env files, local SQLite databases, IDE state and any
  credential-shaped filename out of history. CLAUDE.md 10.1 requires these
  never enter the repo.
- .editorconfig, rustfmt.toml, clippy.toml: formatting and lint policy.
  clippy.toml is written in the lints-file form so lints are not duplicated
  into every crate manifest.

CLAUDE.md is committed verbatim; it is the project spec and the parity
contract for every platform layer.

Co-Authored-By: Claude Code <noreply@anthropic.com>

rustfmt.toml

@@ -0,0 +1,12 @@
+edition = "2021"
+max_width = 100
+
+# Imports are grouped std / external / crate, and sorted within each group.
+imports_granularity = "Crate"
+group_imports = "StdExternalCrate"
+
+# We write a lot of match arms over enums and error plumbing. Keep them tight.
+match_block_trailing_comma = false
+
+# Derive Debug on virtually everything — engine output is logged in the field.
+derive_debug = false