Support mixins referencing other mixins

[PiousCrossten]

Lets one mixin be built out of others. A mixin can now list other mixins under a reserved mixin key, instead of copy-pasting the same cmake-args into every variant.

Addresses #39.

So this becomes possible:

build:
  debug:
    cmake-args: ["-DCMAKE_BUILD_TYPE=Debug"]
  ccache:
    cmake-args: ["-DCMAKE_CXX_COMPILER_LAUNCHER=ccache"]
  dev:
    mixin: ["debug", "ccache"]
    cmake-args: ["-DBUILD_TESTING=ON"]

colcon build --mixin dev applies debug, then ccache, then dev's own arguments - references first, so the mixin doing the referencing gets the last word and can override anything it inherits. Command line arguments still beat all of them.

How it works

colcon_mixin/mixin/order.py is a new standalone module that works out the application order with a depth-first post-order walk. Two details worth flagging for review:

• A mixin reached through more than one path is applied once per path, so the returned list can contain duplicates. That's deliberate — it's what keeps last-one-wins consistent no matter how the graph is shaped.
• Cycle detection only looks at the active recursion path, not everything visited so far, so legitimate re-application isn't mistaken for a cycle.

Circular references, unknown names, and a malformed mixin key each raise their own errthem into ordinary CLI errors rather than tracebacks.

In mixin_argument.py the reserved mixin key is skipped during the overlay, since it's metadata rather than an argument. Mixins that don't use the key behave exactly as before.

Tests

test/test_order.py covers the ordering algorithm and the error cases; test/test_mixin_argument.py adds nested references, scalar overrides, command line precedence, and the error paths through the parser.

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Stacked on #71 - this PR targets that branch, so the diff here shows only the composition work. It'll retarget to master and be rebased once #71 lands.

Claude Opus (via Antigravity) was used for generating test cases and for architecture level testing. The design, implementation and final review are my own.

[knmcguire]

See my comments here on the same PR but in your forked repo but I approve it on my end!

PiousCrossten#1