feat: build host-only doc tools in host config via aspect_bazel_lib platform_transition_binary

[umaucher]

Alternative implementation of #633. Same goal, using the public platform_transition_binary rule from aspect_bazel_lib instead of a hand-rolled transition rule. Opened for side-by-side comparison — only one should be merged.

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 with platform_transition_binary from aspect_bazel_lib, transitioning the wrapped binary to a fixed target_platform (@local_config_platform//:host) and forwarding its executable, runfiles and RunEnvironmentInfo, so bazel run //:docs keeps working unchanged.

• aspect_bazel_lib is already in the module graph via aspect_rules_py; it is declared as a direct bazel_dep (2.16.0) so the rule can be loaded.
• 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.

Comparison with #633

	#633 (custom _host_binary)	this PR (platform_transition_binary)
Code	~35 lines of custom rule + transition	reuse a maintained ruleset rule
Target platform	follows --host_platform dynamically	fixed @local_config_platform//:host (autodetected host)
New dependency	none	direct bazel_dep on aspect_bazel_lib (already transitive)

The dynamic --host_platform behaviour of #633 is more precise when a consumer sets a specific --host_platform that differs from the autodetected host; this PR trades that for less code and a battle-tested rule.

Validation

• Host: bazel build //:docs //:ide_support //:needs_json //:metrics_json — no regression; cquery confirms RunEnvironmentInfo is forwarded and the binary transitions to @local_config_platform//:host.
• Cross: in a consumer with a local override, bazel build --config=per-x86_64-qnx -- //... no longer fails Python toolchain resolution (0 toolchain errors; only the unrelated valid_from skew fixed by Set valid_from attribute for all features score#3064).
• buildifier clean.

[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: 351fe929-c338-4162-985a-ebb9adcb3f93
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: 5.576s
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