Modules can report why they failed

A module had one channel back to its caller: the int returned from
dpm_module_execute, handed through by dpm_execute. Any detail behind
that number could only reach a log, so a caller wanting the reason had
to read output rather than ask for it.

dpm_set_last_error joins the exported API. A module records its reason
on the context handed to its entry point, which belongs to the caller,
and the caller reads it back with dpm_get_last_error.

The two accessors on the context now say what they do to it:
dpm_module_path becomes dpm_get_resolved_module_path, naming the value
it reports rather than the setting it came from, and dpm_last_error
becomes dpm_get_last_error, pairing with the setter.

Path normalization moves to sanitizers.cpp, which holds the conversions
that put a value written by a person into the single form the library
stores it in.

docs/PROSE/CONSUMERS.md

@@ -54,7 +54,7 @@ The root override is what makes chroot builds, image assembly, and sysroot manag
 dpm_module* mod = dpm_require(ctx, "mymodule");
 ```
 
-libdpm-core.so validates the module completely at load; a handle is returned only for a fully valid module. NULL means the module is absent or invalid — `dpm_last_error`(ctx) carries the precise reason. Modules load at most once per context; repeated calls return the same handle.
+libdpm-core.so validates the module completely at load; a handle is returned only for a fully valid module. NULL means the module is absent or invalid — `dpm_get_last_error`(ctx) carries the precise reason. Modules load at most once per context; repeated calls return the same handle.
 
 `dpm_module_info_of` reports what the library saw in the loaded module:
 
@@ -94,12 +94,13 @@ The cursor covers every valid module in the module path; invalid candidates are
 - `dpm_module_info_of`(ctx, mod, out)** — the name, version, and description read from a loaded module.
 - `dpm_config_get`(ctx, module, section, key)** — a value from a module's configuration namespace, or NULL if unset.
 - `dpm_log`(ctx, level, message)** — writes to the context's configured log targets; levels are `DPM_LOG_FATAL` through `DPM_LOG_DEBUG`.
-- `dpm_module_path`(ctx)** — the resolved module directory.
-- `dpm_last_error`(ctx)** — a human-readable description of the most recent failure on the context, or NULL.
+- `dpm_get_resolved_module_path`(ctx)** — the resolved module directory.
+- `dpm_set_last_error`(ctx, msg) — records a failure reason on the context; what a module calls to explain a nonzero return.
+- `dpm_get_last_error`(ctx)** — a human-readable description of the most recent failure on the context, or NULL.
 
 ## Ownership and Errors
 
-Strings returned by the library are owned by the context (or by the module that produced them) and remain valid until `dpm_close`; callers never free them. Functions returning int use 0 for success. Functions returning pointers use NULL for failure, with detail available from `dpm_last_error`.
+Strings returned by the library are owned by the context (or by the module that produced them) and remain valid until `dpm_close`; callers never free them. Functions returning int use 0 for success. Functions returning pointers use NULL for failure, with detail available from `dpm_get_last_error`.
 
 ## Complete Example
 
@@ -116,7 +117,7 @@ int main(void) {
 
     dpm_module* mod = dpm_require(ctx, "info");
     if (!mod) {
-        fprintf(stderr, "%s\n", dpm_last_error(ctx));
+        fprintf(stderr, "%s\n", dpm_get_last_error(ctx));
         dpm_close(ctx);
         return 1;
     }