Improve FeatureFramework quality-of-life ergonomics

[remdui]

Summary

Repository-wide, compatibility-preserving quality-of-life pass across FeatureFramework. The repository was reviewed repeatedly across API, core, toolkit, theme support, Paper, Velocity, integrations, processor, testkits, examples/docs, acceptance fixtures, build scripts, and CI. The changes intentionally avoid architectural churn and focus on recurring developer friction, safer external input, read-only diagnostics, composability, and small allocation/correctness improvements.

Identifiers and public runtime API

• add exception-free FeatureId.isValid(...) / FeatureId.tryParse(...)
• add matching safe parsing to ServerId, ThemeId, and ThemeItemId
• make FeatureCatalog.findByName(...) safe for null/malformed external input
• add FeatureSnapshot.active() / failed()
• add CapabilityRegistry.findCapability(...), requireCapability(...), and hasCapability(...) while retaining reload-safe references
• trim external feature names in the feature command model

Feature, resource, and service composition

• add bulk FeatureCollection.Builder.features(...) and FeatureCollection.size()
• cache immutable effective feature roles instead of rebuilding derived sets on repeated reads
• add exact-key/resource key snapshots plus size/empty introspection to FeatureResourceExtensions
• add availability/type snapshots plus size/empty introspection to InternalServiceRegistry
• add bulk resource-contributor registration to both Paper and Velocity host builders
• add removable FeatureConfigHandler.subscribeReload(...) subscriptions alongside the existing listener API
• add missing-dependency presence/count helpers to DependencyCheckResult

Themes

• cache immutable theme/item list snapshots instead of allocating them repeatedly
• add Theme.size() and ThemeRegistry.size()
• add ThemeRegistry.Builder.include(...)
• allow existing ThemeItem instances and batches to be reused directly through Theme.Builder

Configuration and storage QoL

• add ConfigMap.putAll(...), size(), and isEmpty()
• add ConfigNode map/list shape checks, size(), and optional typed reads
• add ConfigView.contains(...), optional typed reads, atomic putAll(...), and batch putAll(...)
• add feature/local-data path validation helpers without tightening existing accepted paths
• expose YamlFile.path(), optional load-failure diagnostics, and load-failure state
• expose ConfigService.isCached(...), cached file count, and immutable cached-path snapshots

Cache, HTTP, token, and text toolkit QoL

• add typed/raw cache-value lookup, contains/size/empty helpers, deterministic expiration checks, and bulk builder input
• add optional/contains conveniences to FileCacheStore
• fix persisted cache expiration consistency: non-positive expiration timestamps now remain non-expiring in JsonCacheFile, matching CacheValue
• add AsyncHttpTransport.postJson(...)
• add HTTP redirect/client-error/server-error/error classification helpers
• cache the empty MessagePlaceholders instance and add contains/size/empty/map/apply conveniences
• widen TokenService loaders to CompletionStage, add an infinite-token convenience overload, explicit cleanup/clear, namespace, and empty-state helpers
• add TokenResult.isOk() / payloadOptional()
• expose Language.code(), safe case-insensitive Language.fromCode(...), and cache the immutable localizable-language list
• add pagination navigation helpers (hasPrevious, hasNext, isFirst, isLast, previousPage, nextPage)

DataRegistry and DataProvider ergonomics

• add optional snapshot helpers and primitive playerId() convenience to PlayerReference
• add public active identifier lookup and null-safe readiness to PlayerIdentityResolver
• add symmetric username/player-id reference lookups, including async variants, to PlayerReferenceResolver
• make DataProviderResources AutoCloseable
• expose bound-feature state plus immutable connection/ORM identifier snapshots and active counts
• add direct connection/ORM presence checks without exposing mutable resource maps

Testkit

• add empty-handler InterfaceProxy.of(Class) and clearer upfront interface validation
• add test filesystem directory/read helpers
• expand focused API/core/toolkit/cache/config/DataRegistry/DataProvider contract coverage

Compatibility

• additive public API only except internal implementation optimizations/correctness alignment
• no removed or renamed existing methods
• no package moves
• no changed existing return types
• no stricter existing validation or changed defaults
• no lifecycle, platform-threading, scheduling, database, or resource-ownership model changes
• avoided ambiguous overloads such as find(String) next to find(FeatureId)
• retained existing behaviors such as ServerId.optional(...) and mutable ConfigMap collection views
• TokenService's loader widening remains source/binary compatible at erasure and accepts existing CompletableFuture suppliers

Validation

• CI Lint / Checkstyle / ShellCheck: passed
• full Maven verify: passed
• Paper and Velocity platform acceptance: passed
• no open PR conversation comments or unresolved review threads