Draft: Extract instance-identity helpers into gitlab-platform-core

What does this MR do and why?

Move the instance-identity singleton methods out of lib/gitlab.rb into gems/gitlab-platform-core, as a new Gitlab::InstanceIdentity module at lib/gitlab/instance_identity.rb inside that gem: com?, com_except_jh?, com, staging?, canary?, com_and_canary?, com_but_not_canary?, org?, dev_url, community_forum_url, doc_url, promo_host, gl_subdomain?, subdomain_regex, root_domain, staging_com_url, com_url, org_or_com?, dev_or_test_env?, simulate_com? and http_proxy_env?.

Gitlab::InstanceIdentity is deliberately kept in this gem even though it is GitLab-specific, unlike everything else gitlab-platform-core holds. It is the gem's configuration seam into the monolith, not a generic library, and the maintainer agreed it should stay here rather than be left in lib/gitlab.rb.

lib/gitlab.rb does extend InstanceIdentity, so these stay singleton methods on Gitlab and no call site or spec stub changes. The methods still dispatch through self, so stubbing Gitlab.com? still changes what Gitlab.com_and_canary? returns. HTTP_PROXY_ENV_VARS moves with http_proxy_env?. extend does not carry constants, so Gitlab::HTTP_PROXY_ENV_VARS no longer resolves; it had no callers outside lib/gitlab.rb.

Gitlab::Platform::Core::Configuration is the new seam, with three inputs: instance_url, environment and jh. Each takes a plain value or a callable, and callables are resolved on every read rather than once at boot, so stub_config_setting, stub_rails_env and allow(Gitlab).to receive(:jh?) keep working. Reading an input that was never set raises Gitlab::Platform::Core::MissingConfigurationError, which names the input and how to set it.

The monolith wires the inputs in config/initializers/0_gitlab_platform_core.rb. The 0_ prefix puts it ahead of 1_settings.rb, whose top level is the earliest boot-time caller of a moved method, Gitlab.dev_or_test_env?. spec/fast_spec_helper.rb loads that same file directly, because it loads lib/gitlab.rb without running initializers.

lib/gitlab.rb reaches the gem with require_relative into gems/gitlab-platform-core/ rather than by gem name, because scripts/static-analysis, scripts/lib/assets_sha.rb and scripts/cells/application-settings-analysis.rb load it as plain Ruby with no Bundler, and a path: gem can never be resolved that way. For the same reason instance_identity.rb parses GITLAB_SIMULATE_SAAS and CANARY itself instead of calling Gitlab::Utils.to_boolean: requiring gitlab/utils loads active_support/all and action_view, which plain Ruby cannot resolve either. The regexes match to_boolean.

lib/gitlab_edition.rb stays in the monolith. GitlabEdition.root derives the repo root from __dir__, which inside the gem would point at the gem directory. qa/Dockerfile also copies that one file into the QA image, .dockerignore un-ignores it by name, and qa/qa.rb requires it by relative path from a separate bundle that does not include the gem. lib/gitlab.rb keeps maintenance_mode?, version_info, revision, current_milestone, pre_release?, config, host_with_port, process_name and next_rails?.

The matching examples move to gems/gitlab-platform-core/spec/gitlab/instance_identity_spec.rb and run against fake inputs. That spec defines a local stub_env helper and excludes RSpec/EnvMocking for itself, because taking stub_env from gitlab-rspec would pull activerecord and gitlab_quality-test_tooling into this gem's bundle. spec/lib/gitlab_spec.rb keeps a seam test that the wiring resolves on every call, and its now-dead RSpec/NamedSubject entry is pruned from .rubocop_todo/.

The README documents the new "Configuration" section and widens the "Monolith state must come in through an injected seam" line to point at it.

bin/rails runner 'p Gitlab.com?, Gitlab.dev_or_test_env?' returns the same values before and after, in both test and development, with and without GITLAB_SIMULATE_SAAS=1.

Context

Part of #635231, which has the plan, verification and risks for this workstream. The tracking issue for the whole slice is #632651.

This MR is stacked on !260361. Until that one merges, the diff also shows the commits of the MRs below it. Review only the last 1 commit(s) on this branch.

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