Move version compatibility to the consumer; name libdpm-core explicitly

dpm_require no longer takes a minimum version and applies no version
criterion of its own. A handle now means the module is valid, not that
it suits the caller. dpm_module_info_of is added alongside it, reporting
the name, version, description, and minimum-libdpm-core version read at
load, so a consuming module can judge a dependency's version for itself.
The one rule still enforced is the minimum-version handshake, where
libdpm-core is the host and refuses a module that demands a newer library
than the one running.

dpm_module_info_of joins the version script, so the exported surface is
now fourteen symbols under DPM_CORE_1.0.

Separately, the bare word "core" is gone from prose everywhere. It named
both the command-line tool and the library, so every use forced the
reader to guess which. Text now says "the dpm binary" or "libdpm-core".
Identifiers keep their spelling: libdpm-core, core.h, core.conf, the
"core" configuration namespace, dpm_core_version, core_min, DPM_CORE_1.0,
the dpmcore namespace, test_core, core_api.

Three user-visible strings changed with it: the load-refusal message now
reads "requires libdpm-core >= X, running libdpm-core is Y — update
libdpm-core", and the info module's description and help text name the
library. The test asserting on the refusal text was updated to match.

DESIGN.md's terminology line no longer defines "DPM Core" as the CLI,
which was the source of the ambiguity. OVERVIEW.md is restructured around
the three layers a reader meets DPM at — user, developer, filesystem —
so a code-level symbol never appears without saying whose layer it is.
MODULES.md describes the bundled info module as testing and demonstrating
full DPM system functionality rather than as a reference implementation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -5,7 +5,7 @@ set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 # ---------------------------------------------------------------------
-# libdpm-core.so — the core library (C ABI)
+# libdpm-core.so — the library (C ABI)
 # ---------------------------------------------------------------------
 add_library(dpm-core SHARED
         src/context.cpp
@@ -58,7 +58,7 @@ set_target_properties(dpm PROPERTIES
 )
 
 # ---------------------------------------------------------------------
-# info — the bundled module (tests and reports core functionality)
+# info — the bundled module (tests and reports libdpm-core functionality)
 # ---------------------------------------------------------------------
 add_library(info MODULE
         src/bundled-modules/info/info.cpp
@@ -76,7 +76,7 @@ set_target_properties(info PROPERTIES
         LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/modules
 )
 
-# The module bundles with core, so building the CLI builds it too: after a
+# The module bundles with libdpm-core, so building the CLI builds it too: after a
 # clean, running dpm from the build tree finds a module directory that is
 # populated. This orders the build only — the CLI does not link the module.
 add_dependencies(dpm info)
@@ -106,7 +106,7 @@ foreach(fixture good missing_symbols bad_magic lying_manifest core_too_new bad_v
 endforeach()
 
 # ---------------------------------------------------------------------
-# Core API test binary — drives libdpm-core through its public C API
+# API test binary — drives libdpm-core through its public C API
 # ---------------------------------------------------------------------
 add_executable(test_core tests/test_core.cpp)
 
@@ -130,7 +130,7 @@ endforeach()
 add_test(NAME core_api COMMAND test_core)
 
 # ---------------------------------------------------------------------
-# CLI end-to-end tests (the CLI is core's cheapest full-stack client)
+# CLI end-to-end tests (the dpm binary is the cheapest full-stack client)
 # ---------------------------------------------------------------------
 add_test(NAME cli_help COMMAND dpm --help)
 set_tests_properties(cli_help PROPERTIES PASS_REGULAR_EXPRESSION "Usage: dpm")