docs: configuration/construction/verification, runtime_search_dir, the prepare role and the stamp rule (mcpp#701, mcpp#702)

docs/30, and zh: a new section states the three kinds of work a build.mcpp
or the rules it imports do (SPEC-007 §1) -- configuration depends only on
declared inputs, construction is never done while build.mcpp runs, an
incomplete environment is a warning, and one behaviour serves planning and
building alike. `dep_bin` gains the paragraph on a host tool that fails to
build under `emit build-database`: naming it still configures correctly,
calling it while configuring does not. The directive tables and the typed
API table rename runtime-library-dir/runtime_library_dir to
runtime-search-dir/runtime_search_dir and correct what it joins; the worked
section is rewritten to match, with the Qt example fixed to pass `lib/` to
link_search and `bin/` to runtime_search_dir. The action role table gains
`prepare`, spelled with the `mcpp::roles` constants; a new section states
its contract (output_dir, the post-condition, the ordering, the R1.3
warning). The stamp-rule paragraph is rewritten for R2's simple form.

docs/31, and zh: the role table gains `prepare` and points at docs/30's
fuller contract and at SPEC-007.

docs/04, and zh §2.11: `runtime_search_dirs` (not the retiring
`library_dirs`) is named as the field the build-program form joins.

Author: speak-agent

docs/04-mcpp-toml.md

@@ -1473,10 +1473,13 @@ For one compatibility train, `library_dirs` maps only to runtime search,
 `capabilities` maps to required run-phase capability requirements. None of
 these legacy fields creates a provider.
 
-`library_dirs` has a build-program form for a directory only a build.mcpp can
-locate (a vcpkg prefix's `bin/`, a Qt SDK's `bin/`): `mcpp::runtime_library_dir(dir)`
-(2026.9.27.1+, protocol 12; [30 — Build Programs](30-build-mcpp.md)), which
-joins this same field.
+`runtime_search_dirs` has a build-program form for a directory only a
+build.mcpp can locate (a vcpkg prefix's `bin/`, a Qt SDK's `bin/`, a directory
+a `prepare` action populates): `mcpp::runtime_search_dir(dir)` (protocol 12;
+[30 — Build Programs](30-build-mcpp.md)), which joins this same field
+directly — not the legacy `library_dirs` above, which gains no directive of
+its own. The directory need not exist when the program runs; a `prepare`
+action may populate it later, at build time.
 
 `target/<triple>/<fp>/resolution.json` schema 2 stores the RuntimeBinding,
 canonical requirements/providers/artifacts, LinkIntent, platform search