feat(msgs): functional lyrical distro support via feature-selected asset trees

[richardw347]

Summary

The lyrical (and rolling/kilted) Cargo features already exist on the
hiroz crate, but they are non-functional — enabling --features lyrical
silently generates jazzy messages:

• hiroz_codegen::bundled_assets_dir(is_humble) only maps to "humble"/"jazzy".
• hiroz-msgs/build.rs selects packages via an environment probe
  (detect_ros_version() reading AMENT_PREFIX_PATH) plus a hand-maintained
  allow-list (get_all_packages), neither of which is distro-feature aware.
• There is no assets/lyrical/ tree.

This PR makes distro selection real and feature-driven, and finishes the
lyrical support the feature flags already advertise.

Changes

• Distro enum (Humble/Jazzy/Lyrical) selected from Cargo features via
  Distro::from_features(). Defaults to Jazzy when no distro feature is
  set — no behavior change for existing users.
• Asset-tree enumeration replaces the env-probe + allow-list: every package
  directory under hiroz-codegen/assets/<distro>/ (one containing msg/,
  srv/, or action/) is generated. Adding a package is just dropping its
  interface files into the tree — no build.rs edit. (test_msgs /
  test_interface_files stay excluded, as before, because their wstring/Nested
  fixtures don't compile.)
• hiroz_codegen::bundled_assets_dir_for(distro: &str) added; the existing
  bundled_assets_dir(is_humble) now routes through it (kept for compatibility).
• assets/lyrical/ — the stock upstream ROS 2 interface set (25 standard
  packages: std_msgs, sensor_msgs, geometry_msgs, nav_msgs,
  action_msgs, builtin_interfaces, rcl_interfaces, tf2_msgs,
  vision_msgs, type_description_interfaces, …). Shipped to crates.io via the
  existing assets/** include globs in hiroz-codegen/Cargo.toml.

build.rs shrinks substantially (the allow-list + system-package probing +
detect_ros_version are all removed in favor of enumeration).

Verification

cargo build -p hiroz-msgs --no-default-features --features core_msgs,lyrical
  → target ROS distro: lyrical
  → generating 25 packages from assets/lyrical

cargo build -p hiroz-msgs --no-default-features --features core_msgs,jazzy
  → target ROS distro: jazzy
  → generating 24 packages from assets/jazzy   (unchanged)

Notes / open questions for the maintainer

• Default: kept at jazzy to preserve current behavior. Happy to change if
  you'd prefer newest-available or an explicit-required selection.
• Asset set: assets/lyrical/ is stock upstream interfaces only. One
  package, ffmpeg_image_transport_msgs, is community (not ROS-core) — included
  because it round-trips cleanly, but easy to drop from the tree if you'd rather
  keep only core/common_interfaces.
• rolling/kilted: those feature flags still map to no asset tree; this PR
  only adds lyrical. The enumeration mechanism makes adding them trivial later
  (drop an assets/rolling/ tree + a Distro arm).

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