Align Dependabot coverage and add the dev-fast build fragment

[leynos]

Summary

This branch applies Wave 1 of the Rust estate baseline remediation
(Operation Parabellum, phase 2). It aligns
.github/dependabot.yml
with the canonical Dependabot reference: one update stanza per package
ecosystem the repository uses (cargo, rust-toolchain, github-actions), each stanza labelled
dependencies plus its channel label, and GitHub Actions updates batched
into a single pull request via a wildcard group. It also adds the opt-in dev-fast build
fragment with dev-build and dev-test Make targets, signposted in
AGENTS.md.

Review walkthrough

• Start with
  .github/dependabot.yml
  for the ecosystem coverage and labels.
• Then review
  tools/dev-fast/config.toml
  — a copy of the canonical fragment — and the dev-build/dev-test
  targets appended to the
  Makefile.
• Finish with the signposting section appended to
  AGENTS.md.

Validation

• Audited the amended tree with the Concordat rule packages
  dependabot-baseline and rust-dev-fast-baseline: dependabot-baseline compliant, rust-dev-fast-baseline noncompliant.

Notes

• The canonical labels (including the Dependabot channel labels this
  configuration references) were created on the repository ahead of this
  pull request, so no stanza names a missing label.
• The dev-fast fragment is strictly opt-in local tooling: it is only
  applied when passed explicitly with --config, so continuous
  integration, release, coverage, and verification builds are untouched.
  The targets need a nightly toolchain and, on Linux, a mold binary on
  the PATH; repositories still pinned to stable gain the wiring now and
  the capability when Wave 2 moves the pin to nightly.
• Needs a human eye: .cargo/config.toml mentions cranelift.

Summary by Sourcery

Align Dependabot configuration with Rust toolchain coverage and introduce an opt-in fast debug build workflow for local development.

New Features:

• Add nightly-based dev-build and dev-test Make targets that use an opt-in Cargo config for Cranelift and mold-accelerated debug builds.

Enhancements:

• Document the fast development build targets and their constraints in AGENTS.md to guide local developer usage.

Build:

• Add a canonical dev-fast Cargo config fragment for accelerated local debug builds without impacting release or CI builds.

CI:

• Extend Dependabot coverage to manage rust-toolchain updates with appropriate dependency labelling and scheduling.

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[sourcery-ai]

Reviewer's Guide

Aligns Dependabot configuration with the canonical baseline and introduces an opt-in dev-fast build fragment with new Make targets and documentation signposting.

Sequence diagram for opt-in dev-fast build targets

sequenceDiagram
  actor Developer
  participant Makefile
  participant Cargo
  participant dev_fast_config_toml as tools_dev_fast_config_toml
  participant NightlyToolchain
  participant MoldLinker

  Developer->>Makefile: make dev-build
  Makefile->>Cargo: cargo --config DEV_FAST_CONFIG build
  Cargo->>dev_fast_config_toml: load config.toml
  Cargo->>NightlyToolchain: use codegen-backend cranelift
  Cargo->>MoldLinker: use -Clink-arg=-fuse-ld=mold [linux]

  Developer->>Makefile: make dev-test
  Makefile->>Cargo: cargo --config DEV_FAST_CONFIG test
  Cargo->>dev_fast_config_toml: load config.toml
  Cargo->>NightlyToolchain: use codegen-backend cranelift
  Cargo->>MoldLinker: use -Clink-arg=-fuse-ld=mold [linux]

Loading

File-Level Changes

Change	Details	Files
Introduce opt-in accelerated debug build targets wired to a separate Cargo config fragment.	Add DEV_FAST_CONFIG variable defaulting to tools/dev-fast/config.toml. Define dev-build and dev-test phony targets that invoke cargo with the explicit --config fragment. Document that these targets are for Cranelift+mold accelerated debug builds and require nightly.	Makefile
Document the dev-fast build flow and its constraints for contributors.	Add a "Fast development builds" section describing make dev-build and make dev-test. Explain the use of the Cranelift backend and mold linker via tools/dev-fast/config.toml. Clarify that the fragment must not be copied into .cargo/config.toml because it should remain opt-in and leave CI/release builds unaffected.	AGENTS.md
Align Dependabot coverage with the canonical configuration, adding rust-toolchain support.	Add a rust-toolchain update stanza with dependencies and rust-toolchain labels. Configure the rust-toolchain updates to run weekly from the repository root directory. Maintain existing cargo ecosystem configuration consistent with the baseline.	.github/dependabot.yml
Add the canonical dev-fast Cargo configuration fragment for local debug builds.	Create tools/dev-fast/config.toml with an [unstable] section enabling codegen-backend. Configure [profile.dev] to use the Cranelift codegen-backend to speed debug builds. Add a Linux-only target table that sets rustflags to use the mold linker, with comments about interaction with existing rustflags and why this remains outside .cargo/config.toml.	tools/dev-fast/config.toml

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