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>

docs/BUILD.md

@@ -1,4 +1,4 @@
-# Building DPM Core
+# Building libdpm-core and the dpm binary
 
 ## Prerequisites
 
@@ -21,11 +21,11 @@ Artifacts land in:
 
 ```
 <build-dir>/bin/dpm             the CLI
-<build-dir>/lib/libdpm-core.so  the core library
+<build-dir>/lib/libdpm-core.so  the library
 <build-dir>/modules/info.so     the bundled info module
 ```
 
-The dpm binary built here finds the locally built libdpm-core.so on its own — an embedded library search path points it at the lib/ directory next to it in the build tree, so running it directly uses the core you just built with nothing to set up first. This is only the default: LD_LIBRARY_PATH takes precedence over the embedded path, so the binary can be pointed at any other core, including the system-installed one.
+The dpm binary built here finds the locally built libdpm-core.so on its own — an embedded library search path points it at the lib/ directory next to it in the build tree, so running it directly uses the library you just built with nothing to set up first. This is only the default: LD_LIBRARY_PATH takes precedence over the embedded path, so the binary can be pointed at any other libdpm-core, including the system-installed one.
 
 ### Running the tests
 
@@ -33,7 +33,7 @@ The dpm binary built here finds the locally built libdpm-core.so on its own —
 ctest --test-dir <build-dir> --output-on-failure
 ```
 
-This runs the core API test binary (which exercises the full load-time validation matrix against the fixture modules in tests/fixtures/) and the CLI end-to-end tests.
+This runs the API test binary (which exercises the full load-time validation matrix against the fixture modules in tests/fixtures/) and the CLI end-to-end tests.
 
 ### Running the CLI from the build tree
 
@@ -60,11 +60,11 @@ This is the packaging flow: configure once, build once, test what was built, ins
 
 ```
 bin/dpm                   the CLI
-lib/libdpm-core.so        the core library
+lib/libdpm-core.so        the library
 lib/dpm/modules/info.so   the bundled info module
 include/dpm/              the public header
 ```
 
-Core configuration installs to /etc/dpm/conf.d/core.conf regardless of prefix.
+The libdpm-core configuration installs to /etc/dpm/conf.d/core.conf regardless of prefix.