Implement topological_sort_levels

[joemalle]

This phab implements issue #240 . Please note that I'm not a frequent contributor, and I may have made some noob mistakes. Also, I used Claude to write this. LMK if that's a foul.

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

Adds functions to toposort into levels. There's a lower level function that uses a property map to output the vertex levels and a vector shorthand.

Motivation

Fixes issue #240

Testing

I added new tests. Here's what I ran

$ (cd libs/graph/test && ../../../b2 topological_sort_levels_test)
Performing configuration checks

    - default address-model    : 64-bit (cached) [1]
    - default architecture     : x86 (cached) [1]
    - symlinks supported       : yes (cached)

[1] gcc-10
...patience...
...patience...
...found 2402 targets...
...updating 4 targets...
gcc.compile.c++ ../../../bin.v2/libs/graph/test/topological_sort_levels_test.test/gcc-10/debug/x86_64/threading-multi/visibility-hidden/topological_sort_levels_test.o
gcc.link ../../../bin.v2/libs/graph/test/topological_sort_levels_test.test/gcc-10/debug/x86_64/threading-multi/visibility-hidden/topological_sort_levels_test
testing.capture-output ../../../bin.v2/libs/graph/test/topological_sort_levels_test.test/gcc-10/debug/x86_64/threading-multi/visibility-hidden/topological_sort_levels_test.run
**passed** ../../../bin.v2/libs/graph/test/topological_sort_levels_test.test/gcc-10/debug/x86_64/threading-multi/visibility-hidden/topological_sort_levels_test.test

...updated 4 targets...

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[jeremy-murphy]

Also, I used Claude to write this. LMK if that's a foul.

It's not a foul, and it is important to note, so thank you.

[jeremy-murphy]

Could you please go into more detail on:

• why this algorithm is useful (what problems does it solve, references to literature, etc)
• why you choose this implementation, how it relates to the existing topological sort algorithm and why, for example, is it not an enhancement of the existing algorithm?

Thanks, cheers.

[Becheler]

Thank you @joemalle for the PR and @jeremy-murphy for the comment. Feel free to edit your own PR first message 😄
Thanks again for shipping this PR on a Sunday so quickly after I reached out 🚀

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 27098118816 vs develop run 27120109824 (724d8f3c9a).

Job	Baseline	After	Delta
macos (clang, 14)	75	75	0
macos (clang, 17)	70	70	0
macos (clang, 20)	70	70	0
ubuntu (clang-19, 14)	75	75	0
ubuntu (clang-19, 17)	75	75	0
ubuntu (clang-19, 20)	75	75	0
ubuntu (clang-19, 23)	75	75	0
ubuntu (gcc-14, 14)	51	51	0
ubuntu (gcc-14, 17)	51	51	0
ubuntu (gcc-14, 20)	51	51	0
ubuntu (gcc-14, 23)	51	51	0
windows_msvc_14_3 (msvc-14.3)	1226	1232	+6

[Becheler]

Note 1 : the +6 warnings on msvc are not an error of the test or header, it's due to Boost.PropertyMap lib forcing a size_t -> unsigned -> size_t. I opened a PR on Boost.PropertyMap repo that should fix this warning (and others) : boostorg/property_map#37

Note 2 Boost.PropertyMap was community maitained, I jumped in but the CI is on fire, we should not condition the present topological_sort_levels on those 6 warnings that are not @joemalle responsibility 🥲