Align Dependabot coverage and add the dev-fast build fragment

.github/dependabot.yml

@@ -28,3 +28,10 @@ updates:
     cooldown:
       default-days: 7
       semver-major-days: 14
+  - package-ecosystem: rust-toolchain
+    directory: /
+    labels:
+      - dependencies
+      - rust-toolchain
+    schedule:
+      interval: weekly

AGENTS.md

@@ -378,3 +378,12 @@ The following tooling is available in this environment:
 
 These practices help maintain a high-quality codebase and facilitate
 collaboration.
+
+## Fast development builds
+
+`make dev-build` and `make dev-test` compile with the opt-in Cranelift
+backend and the mold linker configured in `tools/dev-fast/config.toml`.
+They require a nightly toolchain and, on Linux, a `mold` binary on the
+`PATH`. The fragment is passed explicitly with `--config`, so release,
+coverage, and verification builds are unaffected; never copy its contents
+into `.cargo/config.toml`, which Cargo applies to every build.

Cargo.toml

@@ -79,3 +79,7 @@ missing_docs                        = "deny"
 
 [lints.rustdoc]
 missing_crate_level_docs            = "deny"
+
+[dev-dependencies]
+camino = "1.2.5"
+rstest = "0.26.1"

Makefile

@@ -126,3 +126,14 @@ rust-audit: ## Audit the Rust workspace for known vulnerabilities
 help: ## Show available targets
 	@grep -E '^[a-zA-Z_-]+:.*?##' $(MAKEFILE_LIST) | \
 	awk 'BEGIN {FS=":"; printf "Available targets:\n"} {printf "  %-20s %s\n", $$1, $$2}'
+
+# Opt-in accelerated debug builds (Cranelift + mold); requires a nightly
+# toolchain. See AGENTS.md and tools/dev-fast/config.toml.
+DEV_FAST_CONFIG ?= tools/dev-fast/config.toml
+
+.PHONY: dev-build dev-test
+dev-build: ## Build debug binaries with Cranelift and mold
+	$(CARGO) --config "$(DEV_FAST_CONFIG)" build
+
+dev-test: ## Run tests with Cranelift and mold
+	$(CARGO) --config "$(DEV_FAST_CONFIG)" test

docs/developers-guide.md

@@ -59,6 +59,31 @@ LLVM-compatible linker behaviour.
 Install `clang`, `lld`, `mold`, `python3`, and `cargo-audit` before running the
 full generated workflow locally on Linux.
 
+## Fast development builds
+
+`make dev-build` and `make dev-test` offer an opt-in, faster iteration loop
+for local debug work. `dev-build` compiles debug binaries and `dev-test` runs
+the test suite; both use the Cranelift codegen backend and the mold linker
+configured in `tools/dev-fast/config.toml`.
+
+The `DEV_FAST_CONFIG` variable names that fragment, defaulting to
+`tools/dev-fast/config.toml`, and both targets pass it to Cargo explicitly
+with `--config "$(DEV_FAST_CONFIG)"`. Cargo never auto-discovers this
+fragment; it takes effect only when a target invokes it directly, so other
+`make` targets are unaffected.
+
+Using the fragment requires a nightly toolchain, because the Cranelift
+codegen backend is unstable. On Linux it also requires the mold linker on
+`PATH`; the fragment gates the linker flag behind a `target_os = "linux"`
+`cfg` table, so other platforms fall back to their default linker.
+
+Cranelift configuration must never be copied into `.cargo/config.toml`.
+Cargo auto-discovers that file and applies it to every invocation, which
+would silently degrade release, coverage, and verification builds to the
+faster but less optimizing backend. Keep the fast-build configuration
+isolated in `tools/dev-fast/config.toml` and reach it only through
+`make dev-build` and `make dev-test`.
+
 ## Spelling policy
 
 The tracked `typos.toml` is generated from the shared estate dictionary and the