[schmitch]

Fixes #444

Part of #442. This PR is stacked on #456 (#443), which moves the format crates to crates/formats/. Merge that one first; GitHub then retargets this one to main.

Changes

The descriptor. Each format describes itself with a 'static value that FormatPlugin::descriptor() returns. The new types live in forge-plugin-api, next to RepositoryKind:

• Descriptor: id, name, version, description, mount, kinds, storage, assets;
• Storage;
• Assets, with &'static str fields and Assets::DEFAULT in place of the serde default;
• DEFAULT_MOUNT.

NuGet and raw each define pub const DESCRIPTOR next to their impl FormatPlugin, with version: env!("CARGO_PKG_VERSION"). A typo in a key or a kind now fails the build.

The host side.

• Plugin::builtin(implementation) takes only the format.
• Plugin::descriptor() replaces Plugin::manifest().
• Plugin::id() returns descriptor().id without leaking a copy.
• RepositoryKind::as_str() saves the format registry its own mapping.

Checks. The startup parser's checks are now forge_plugin::descriptor::check:

• the id's spelling and RESERVED_IDS;
• the mount and RESERVED_MOUNTS, together with the existing test that every web UI page is reserved;
• the kinds, each once;
• the asset names and content types.

check also refuses two formats with the same id. A new test in forge-server, plugins::tests::the_builtin_formats_describe_themselves_well, runs it over plugins::builtin(). It also shows that check refuses a malformed or reserved id, a second format with a built-in's id, and invalid asset names. The probe plugin in plugin_contract.rs checks its own descriptor the same way.

Removed:

• plugin.toml from both crates;
• plugin_toml!, PLUGIN_TOML, Manifest::parse and manifest.rs;
• PluginError and SUPPORTED_APIS;
• forge_plugin_api::API and the api key;
• the plugin_toml_names_this_crate test;
• the "keep it equal to version in plugin.toml" comments;
• forge-plugin's toml dependency, plus its now unused serde and thiserror.

Docs. crates/formats/README.md and Layout and Writing a plugin in docs/format-plugins.md now show the descriptor. The status note no longer mentions a declared contract version.

The new doc example on Descriptor is no_run. test-parallel.sh records every test binary cargo would run, and that includes a doctest's temporary binary, which no longer exists when the script runs it. That's why the other doc examples in the repo don't run either.

No changelog fragment, since nothing changes for anyone running Forge.

Checks

• cargo fmt --check and cargo clippy --workspace --all-targets -- -D warnings pass.
• test-parallel.sh --workspace: 105 of 106 test binaries passed, including the existing admin tests of GET /admin/formats, unchanged. The 106th was the doctest's temporary binary described above. With the example no_run, test-parallel.sh over the plugin crates passes, the doctest included.
• cargo tree -p forge-plugin shows no toml.
• git grep -n 'plugin\.toml\|plugin_toml\|PLUGIN_TOML' finds nothing outside CHANGELOG.md and docs/changes/.

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