fix(check-platform-reqs): align with Composer's CheckPlatformReqsCommand flow

Mirror `CheckPlatformReqsCommand::execute` end-to-end: build an
`InstalledRepoLite` from lock/installed plus the root and the real
PlatformRepository, ksort the combined `$requires`, and run the
candidate matching loop with `findPackagesWithReplacersAndProviders`
so an installed package that `provide`s or `replace`s a platform name
(e.g. `symfony/polyfill-mbstring` providing `ext-mbstring`) is now
recognised as satisfying the requirement.

Fixes the JSON output schema to match Composer:
`failed_requirement` is the `{source, type, target, constraint}`
object (or null), `provider` is the bare "provided by …" string (or
null), and `status` is the unwrapped `success`/`failed`/`missing`.
Also switches `--format` to a clap `value_parser` and replaces the
"No installed packages found" hard error with Composer's warn-then-
proceed path so an empty lock yields `[]` and exit 0.

Adds `mozart_core::installer::InstalledCandidate` plus
`InstalledRepoLite::add_candidate` /
`find_with_replacers_and_providers` as the shared substrate for
future commands (`depends`, `prohibits`, `audit`) that need the same
provider/replacer index.

1 files changed, 8 insertions, 0 deletions

diff --git a/crates/mozart-core/src/installer/mod.rs b/crates/mozart-core/src/installer/mod.rs
new file mode 100644
index 0000000..8572627
--- /dev/null
+++ b/crates/mozart-core/src/installer/mod.rs
@@ -0,0 +1,8 @@
+pub mod installed_repo;
+pub mod suggested_packages_reporter;
+
+pub use installed_repo::{InstalledCandidate, InstalledRepoLite};
+pub use suggested_packages_reporter::{
+    HasSuggests, MODE_BY_PACKAGE, MODE_BY_SUGGESTION, MODE_LIST, RootInfo,
+    SuggestedPackagesReporter, Suggestion,
+};