2026.10.1.2: a pack states its build, every drive takes the job count, and a unit's compile does not depend on the member selection (#751, #753)

[speak-agent]

Implements .agents/docs/2026-10-01-pack-drive-and-selection-independent-compile-design.md (revision 2; D1 to D4 accepted as recommended, D5: one pull request). Resolves #751 and #753.

A. Every ninja drive of a plan is configured from the plan (#753)

• The backend takes -j from plan.scheduleNinjaJobs and reclaims stale detach-codegen tokens once per build directory per command. BuildOptions::parallelJobs is removed. Measured on 2026.10.1.1: [build] jobs = 3 reached ninja from mcpp build only; mcpp test and mcpp pack ran ninja's default.
• A drive is reported whenever the command opened the report (BuildOptions::Report::Region, the default); mcpp test's own drives pass Caller.
• mcpp pack builds every configuration group, writes one Finished, then stages and dispatches; the dispatch pass is reported; a failed drive is reported once (report_failed_drive); the global dependency cache is filled after a pack's build (populate_dependency_cache).

B. A unit's compile does not depend on the member selection (#751)

Three facts about the whole graph reached other members' commands, so each switch between --workspace and -p recompiled them:

1. BMI placement and importer flags (W10's census of module names over the graph). Now every package's BMIs lie below its own directory except the root's (the [bug] 对象路径按「父目录名+文件名」折叠,不同目录同名源文件冲突(multiple rules generate *.ddi) #233 rule), and each unit reads one module map of the modules it reaches through its imports: -fmodule-mapper= on GCC, @<build dir>/modmap/<package>-<hash>.modmap on clang and MSVC. A project whose modules are all its own is byte-identical. The global cache now serves qualified BMIs (DepArtifacts::bmiBuildRel).
2. A file a member lists from outside its directory was owned by the virtual root, whose object census depended on the selection (found by the whole-argument-list criterion; this is GalTranslPP's boost.ixx case).
3. needsPic was a census of shared link units. Position independence now follows the target: every unit of an ELF target that is not freestanding.

Criteria

• Unit: tests/unit/test_module_address.cpp (5 tests), ObjectAddress.AMembersOutsideSourceIsImmuneToAnotherMemberListingIt (fails without the fix).
• e2e 871 (pack report, JSON, jobs = 1 no-overlap for build/test/pack) and 872 (no recompile across selections, per-source argument-list equality of the build databases, the feature-union exception, a cache-served dependency module). Both fail on 2026.10.1.1 and pass on this branch, locally under GCC 16.1.0 and clang 22.1.8.
• Restated: e2e 09, 847 A/G, 849 B (a dependency's BMI path gains its package directory).

Measurements before implementation (design section 8)

• M1: relocated BMIs load through a complete map on GCC 16.1.0 and clang 22.1.8.
• M2: clangd 22.1.8 expands @file relative to the database directory; mcppls expands response files and drops module mechanics, including an unexpanded @*.modmap.