[octalide]

Closes #4834

An open buffer under a project now composes its build request through the same code as the project snapshot, so the editor has one composition of a project's request.

Plan

• project_build composes a project's request from its manifest and a ProjectSelection with request.compose for the check goal, as mach check does. snapshot_load and the buffer path (proj_ensure) both call it.
• AnalysisRequest.build becomes selection: ProjectSelection. The loaded project is keyed on that selection (ProjKey holds owned copies of artifact, target and profile), and its request borrows those copies. That way the request stays the composed one across analyses, and the post-analysis reset to request.defaults() and the driver.reconfigure on reuse are deleted.
• Regenerated doc/mach/lang/editor.md.

Target resolution is not involved. The selection's target is passed through as the snapshot already does, so manifest.resolve_declared is not touched.

Decisions

• A file under no manifest keeps explicit options. A loose file has no profile to compose from, so AnalysisRequest keeps standalone_build (default request.defaults()) beside standalone_target. A project's options are never taken from it. The one test that set build.opt on a standalone buffer now sets standalone_build.opt.
• Breaking, labelled major. Composition runs profiles_check and the selected profile's pass, skip and relax lists. So an open buffer's analysis now refuses a manifest that mach check refuses, such as an unknown pass name in any profile, where the default request accepted it.
• Key on the selection, not the composed request. The composed request depends only on the manifest and the selection, and the manifest is read again whenever the project reloads.

Ran

b check . was clean. b test . --filter lang.editor --timeout 5m passed 50 of 50. b fmt --check . and the committed-reference diff were also clean.