chore: route rustc through sccache for cold-build wins

Adds .cargo/config.toml setting `rustc-wrapper = "sccache"` so cold
rebuilds (CI, fresh checkouts, post-`cargo clean`) replay previously-
compiled crates from disk instead of recompiling. Warm incremental
builds are unaffected — cargo's own target/ cache dominates there.

Cache lives at `.sccache-cache/` inside the project (gitignored). The
[env] entry uses `force = false` so a developer-set $SCCACHE_DIR in
their shell wins, matching whichever directory the sccache daemon
already adopted.

Requires sccache on PATH. Install: `pacman -S sccache`,
`brew install sccache`, or `cargo install sccache --locked`. Bypass
without editing: `RUSTC_WRAPPER= cargo build`.

.cargo/config.toml

@@ -0,0 +1,31 @@
+# Project-wide cargo configuration.
+#
+# Routes every rustc invocation through `sccache` so cold rebuilds and
+# fresh checkouts (CI, new dev box, after a `cargo clean`) replay
+# previously-compiled crates from a local on-disk cache rather than
+# recompiling them. Warm incremental builds still go through cargo's
+# own `target/` cache, which dominates locally — sccache buys you the
+# big wins on cold paths.
+#
+# Requires sccache on PATH. Install it once per machine:
+#
+#   Arch     :  pacman -S sccache
+#   macOS    :  brew install sccache
+#   Cargo    :  cargo install sccache --locked
+#
+# Without sccache the build fails with "rustc-wrapper not found". To
+# bypass this config without editing the file, prepend
+# `RUSTC_WRAPPER= ` (empty value) to your cargo command:
+#
+#   RUSTC_WRAPPER= cargo build
+#
+[build]
+rustc-wrapper = "sccache"
+
+# Project-local cache so the shared dev box (or a Docker volume) keeps
+# the artefacts isolated per checkout instead of mixing them in
+# `~/.cache/sccache`. Set with `force = false` so a developer-set
+# `SCCACHE_DIR` in their shell wins — important because the sccache
+# daemon, once started, sticks with whichever directory it saw first.
+[env]
+SCCACHE_DIR = { value = ".sccache-cache", relative = true, force = false }

.gitignore

@@ -1,4 +1,5 @@
 /target
+/.sccache-cache
 *.db
 *.db-shm
 *.db-wal