chore(release): prepare v0.4.2

[Reddimus]

Sets the version to 0.4.2 in include/spc/version.hpp, dates the changelog section, and points the README's GIT_TAG at v0.4.2, which make test-consumers checks. The find_package(spc 0.4 REQUIRED) line stays, because 0.4.x is compatible.

v0.4.2 contains the performance and correctness work measured in each PR: #16 and #28 (the storm report list is sized once, and only for real reports), #19 (numeric strings parse the same way on every platform), #21 (the typed ArcGIS queries parse each page once, about half the CPU and allocations), #24 (the parse_* functions stay inside their string_view), #25 (error docs), plus the CI and test-tooling changes in #22, #26, and #29. No public API changes.

After merge, tagging v0.4.2 runs the release workflow. It checks the tag against version.hpp, runs the tests and consumer checks, and publishes the changelog section as the release notes.

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