feat: build host-only doc tools in host config under cross --platforms

[umaucher]

Why

The runnable documentation targets created by docs() — docs, docs_combo, docs_check, docs_link_check, live_preview, live_preview_combo_experimental, ide_support — are host tools that run Python/Sphinx on the build machine. When a consumer builds with a cross --platforms (e.g. --config=per-x86_64-qnx -- //...), these top-level targets get configured for the target platform and fail Python toolchain resolution (No matching toolchains ... tools/python), breaking the whole build in cross-compiling consumers (persistency, reference_integration).

What

Each host-only run target is wrapped in a small rule (_host_binary) whose incoming transition pins //command_line_option:platforms to whatever --host_platform is set to. The targets therefore always build and run in the host configuration regardless of the top-level --platforms.

• The wrapper forwards the executable (via symlink), runfiles, and RunEnvironmentInfo, so bazel run //:docs (and the env-carrying targets) keep working unchanged.
• The transition follows --host_platform dynamically rather than hardcoding a platform, so it works for any consumer's host.
• sphinx_build is only ever consumed as a tool (already built in the exec/host config), so it needs no transition; it is tagged manual so a wildcard bazel build //... under a cross platform skips it instead of trying to configure it for the target platform.

Validation

• Host: bazel build //:docs //:needs_json //:metrics_json, bazel run //:ide_support — no regression.
• Cross: in a consumer with a local override, bazel build --config=per-x86_64-qnx -- //... no longer fails Python toolchain resolution — the wrapped run targets transition to the host config, while sphinx_build (a manual tool) is skipped by the wildcard and built only in the exec/host config when consumed by needs_json. cquery --transitions=lite confirms the wrapped targets transition to the host config. (The only remaining consumer failure is the unrelated valid_from metamodel skew fixed by Set valid_from attribute for all features score#3064.)
• buildifier clean.

No tracking issue.

[github-actions]

License Check Results

🚀 The license check job ran with the Bazel command:

bazel run --lockfile_mode=error //src:license-check

Status: ⚠️ Needs Review

Click to expand output

[License Check Output]
Extracting Bazel installation...
Starting local Bazel server (8.6.0) and connecting to it...
INFO: Invocation ID: 1364cb45-f071-485b-9bf6-779ced10787c
Computing main repo mapping: 
Loading: 
Loading: 0 packages loaded
Loading: 0 packages loaded
Loading: 0 packages loaded
    currently loading: src
WARNING: Target pattern parsing failed.
ERROR: Skipping '//src:license-check': no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
ERROR: no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
INFO: Elapsed time: 5.467s
INFO: 0 processes.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target

[github-actions]

The created documentation from the pull request is available at: docu-html

[umaucher]

requires

• Set valid_from attribute for all features score#3064