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>

clippy.toml

@@ -0,0 +1,26 @@
+# Clippy is run with `-D warnings` in CI (CLAUDE.md §10.4), so every lint we
+# enable here is a build failure. Only turn on lints we are willing to fix.
+
+[lints.rust]
+# Silent truncation of a u64 millisecond count into a u32 is a real bug class in
+# this codebase (usage counters, monotonic deltas). Make it impossible to miss.
+integer_conversions = "deny"
+
+[lints.clippy]
+# Engine code is mostly pure data transformation; `&self` everywhere reads better
+# than `&mut self` and avoids accidental aliasing bugs across the FFI boundary.
+needless_pass_by_ref_mut = "warn"
+
+# Large enum variants waste stack in the hot evaluate() path.
+large_enum_variant = "warn"
+
+# Every OS call returns Result; we handle them all, but catch `.unwrap()` slips.
+unwrap_used = "deny"
+expect_used = "deny"
+
+# Panicking in release builds is a last resort — see CLAUDE.md §0.5.
+panic = "deny"
+
+# Prefer clarity over micro-optimisation; perf budgets in §5 are measured with
+# real benchmarks, not by guessing at clever rewrites.
+too_many_arguments = "warn"