2026.9.29.5: a workspace's build programs are reused across selections, the build database plans by configuration, and a build reports each step once with its outcome while a status line states the build

[speak-agent]

The validation project's post-release run of 2026.9.29.4 showed five defects
in the commands around the workspace build (design document section 17.1).

• D1: a member program's graph document listed every requester in the plan,
  the virtual root included, so the program's re-run key followed the
  selection and -p, pack and emit reran the programs a --workspace build ran.
  The document lists the requests made inside the program's closure
  (graph_package_entry takes the closure's packages).
• The members' programs run dependencies first (depth-first over the plan's
  requests; a cycle skips its closing edge); they ran in discovery order.
• D2: mcpp emit build-database and mcpp build --configure-only plan a
  workspace by configuration, each member's tests included
  (BuildOverrides::member_targets), so a package the members share is described
  once per configuration; members that are programs and their tests are
  described as such. A configuration whose plan fails is planned member by
  member, so a member's failure affects that member only. Set names carry a
  configuration prefix only in a document of several configurations; a set and
  a compile command (file and output) are described once (SPEC-005 v1.6).
• A command that plans several configurations publishes the root
  compile_commands.json once, as the union of their databases; each
  configuration replaced it, a race under concurrent groups.
• D3: the build program status lines name the package.
• D4: a selected member is announced by its directory in a --workspace build.
• D5: mcpp pack reports more than eight outputs by the entry each lies in
  below their common directory, with a count; --verbose and
  --message-format json name every output.
• e2e 839, 840, 841; docs/07, docs/10, docs/30 in both languages; SPEC-005 v1.6;
  version 2026.9.29.5.

The cross-verification of this pull request then printed nothing for 40
minutes between the last Compiling line and Finished. The second commit
makes a build report each step once, with its outcome, and state the build
while it runs (.agents/docs/2026-09-29-build-progress-display-design.md):

• ninja is read as it runs: its status lines give the counts, its log says
  which step finished and when, a step record beside build.ninja names each
  step's package, and the action wrapper reports a check or prepare action's
  start.
• A package's line is written when every step of it has run (done <span>),
  or when the build ends; requested packages are listed, dependencies folded;
  --verbose lists everything and each step.
• A build program has one line (ran, cached, failed).
• A terminal shows the running lines and one status line below the output,
  e.g. Building 612/1203 · 14:32 · gpp.gui: vcpkg install 6:10; a log
  repeats the status line after a minute of silence.
• A failed step is reported when it fails; Finished states the whole
  command's time and how it was spent.
• Terminal detection on macOS and Windows; UTF-16 console output on Windows;
  mcpp.log's verbose records reach the terminal through mcpp.ui's writer.
• e2e 842, 843; unit tests; docs/00, 09, 30, 40 in both languages.

Each new e2e fails on the published 2026.9.29.4 and passes here. Reviewed from the architecture, stability, compatibility, semantics, readability, simplicity, performance and cross-platform angles; the review's findings are addressed in this change.

Merged only after both gates pass: this pull request's CI, and the validation project's cross-verification with the mcpp built from this branch.

Refs #734.