refactor: drop direct dependency on Boost.Spirit (graphviz parser)

[Becheler]

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

• graphviz.hpp: removed both Boost.Spirit code paths in the two read_graphviz overloads (they now always call read_graphviz_new)
• dropped the unconditional classic_multi_pass Spirit include.
• BOOST_GRAPH_USE_SPIRIT_PARSER still recognized, but now emits a BOOST_PRAGMA_MESSAGE deprecation notice and redirects to the default parser instead of selecting Spirit.
• Deleted detail/read_graphviz_spirit.hpp (the Spirit DOT reader that pulled in 10 boost/spirit headers).
• test/isomorphism.cpp: removed the incidental #define BOOST_GRAPH_USE_SPIRIT_PARSER
• build.jam and CMakeLists.txt dropped the spirit dependency.

Motivation

The spirit-based graphviz parser has been non default, undocumented and untested for 16 years.
The Boost.Spirit library has 28 direct boost dependencies, and 54 indirect, effectively dragging most of Boost in BGL: https://alandefreitas.github.io/boostdep_graph/libs/spirit.html

This PR removes its implementation, effectively dropping spirit dependency and its direct+transitive chain.

The default parser uses Boost.Regex, that has a much better dependency chain: https://alandefreitas.github.io/boostdep_graph/libs/regex.html

Note: spirit is not effectively removed from the transitive dependencies chain, as Boost.Serialization pulls it in.
A next PR will drop Boost.Serialization for Boost.Core.Serialization.

Testing

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.

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 31210846512 vs develop run 31207793516 (0478c2d0d5).

Job	Baseline	After	Delta
macos (clang, 14)	681	681	0
macos (clang, 17)	641	641	0
macos (clang, 20)	641	641	0
ubuntu (clang-19, 14)	681	681	0
ubuntu (clang-19, 17)	641	641	0
ubuntu (clang-19, 20)	641	641	0
ubuntu (clang-19, 23)	641	641	0
ubuntu (gcc-14, 14)	827	827	0
ubuntu (gcc-14, 17)	949	949	0
ubuntu (gcc-14, 20)	949	949	0
ubuntu (gcc-14, 23)	949	949	0
windows_msvc_14_3 (msvc-14.3)	971	971	0

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 31210846404 vs develop run 31207793080 (0478c2d0d5).

Header-inclusion weights (graph files pulling each direct dependency in):

Dependency	develop	PR	Δ
spirit	2	0	-2
config	94	93	-1
core	13	12	-1
function	2	1	-1
property_map	79	78	-1
throw_exception	11	10	-1
type_traits	41	40	-1

Transitive Boost modules: 68 → 68 (0)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.66%. Comparing base (e8e3f01) to head (0478c2d).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@ Coverage Diff @@## develop #533 +/- ##
===========================================
+ Coverage 92.45% 92.66% +0.20% 
===========================================
Files 392 391 -1 Lines 28214 28007 -207 Branches 8008 7916 -92 ===========================================
- Hits 26086 25953 -133 + Misses 2024 1951 -73 + Partials 104 103 -1 

Files with missing lines	Coverage Δ
include/boost/graph/graphviz.hpp	96.44% <ø> (-0.06%)	⬇️
test/isomorphism.cpp	64.59% <ø> (ø)

... and 3 files with indirect coverage changes

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