feat: implement shared backend interface and selection model

[niteshpurohit]

• Added a shared Karya::Backend contract with identifier normalization, backend descriptors, lifecycle hooks, and the minimal InMemory backend wrapper.
• Implemented Karya::Backend::Selection as the shared backend identifier model for in_memory, sqlite, redis, postgres, and mysql.
• Kept the contract identifier-based: removed backend classification/capability reporting from this PR and aligned docs to the implemented selection model.
• Preserved direct queue-store validation semantics in Backend::InMemory#build_queue_store and hardened the supervisor signal path so runtime phase writes are best-effort only after the runtime claim exists.
• Added and updated Ruby, RBS, docs, and spec coverage for the shared backend interface and selection model.

closes: #105

[Copilot]

Pull request overview

This PR introduces the first shared backend contract in core/karya, adding a backend namespace that describes backend identity, capability reporting, lifecycle hooks, and identifier normalization on top of the existing queue-store layer. It also adds mirrored RBS and focused specs so later adapter/framework work can target one common backend selection model instead of ad hoc conventions.

Changes:

• Add Karya::Backend runtime types for backend selection, descriptors, capabilities, a base contract, and an InMemory backend wrapper.
• Add mirrored RBS for backend identifiers/classifications and the new backend classes/modules.
• Add backend-focused specs and update review/RBS instructions to tighten how accepted input surfaces should be modeled.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
core/karya/spec/karya/backend/selection_spec.rb	Adds specs for backend identifier normalization, classification, and support checks.
core/karya/spec/karya/backend/in_memory_spec.rb	Adds specs for the new in-memory backend descriptor, queue-store construction, and lifecycle hooks.
core/karya/spec/karya/backend/descriptor_spec.rb	Adds specs for descriptor normalization and validation behavior.
core/karya/spec/karya/backend/capabilities_spec.rb	Adds specs for capability flag validation and parity exception handling.
core/karya/spec/karya/backend/base_spec.rb	Adds specs for the shared backend base contract and descriptor delegation.
core/karya/sig/karya/backend/selection.rbs	Declares RBS for backend selection normalization and classification helpers.
core/karya/sig/karya/backend/in_memory.rbs	Declares RBS for the concrete in-memory backend wrapper.
core/karya/sig/karya/backend/descriptor.rbs	Declares RBS for backend descriptors.
core/karya/sig/karya/backend/capabilities.rbs	Declares RBS for backend capability metadata.
core/karya/sig/karya/backend/base.rbs	Declares the shared backend interface in RBS.
core/karya/sig/karya/backend.rbs	Adds top-level backend namespace and backend-selection error classes to RBS.
core/karya/sig/karya.rbs	Adds backend identifier/classification type aliases used by the new backend API.
core/karya/lib/karya/backend/selection.rb	Implements backend identifier aliasing, normalization, and deployment classification helpers.
core/karya/lib/karya/backend/in_memory.rb	Adds the first concrete backend wrapper around QueueStore::InMemory.
core/karya/lib/karya/backend/descriptor.rb	Implements backend descriptor normalization and validation.
core/karya/lib/karya/backend/capabilities.rb	Implements immutable-ish backend capability metadata and validation.
core/karya/lib/karya/backend/base.rb	Introduces the shared backend contract and default lifecycle hooks.
core/karya/lib/karya/backend.rb	Adds the backend namespace, errors, and autoload wiring.
core/karya/lib/karya.rb	Loads the new backend namespace from the main library entrypoint.
.github/instructions/review.instructions.md	Updates review guidance for accepted input surfaces and shared-contract RBS exactness.
.github/instructions/core-karya-rbs.instructions.md	Updates Core Karya RBS instructions with backend-input and shared-contract typing rules.

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Copilot reviewed 21 out of 21 changed files in this pull request and generated 3 comments.

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Copilot reviewed 21 out of 21 changed files in this pull request and generated 4 comments.

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[Copilot]

Pull request overview

Copilot reviewed 21 out of 21 changed files in this pull request and generated 5 comments.

[Copilot]

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Copilot reviewed 21 out of 21 changed files in this pull request and generated 4 comments.

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Comments suppressed due to low confidence (1)

docs/pages/backends.md:33

• This section is now titled "Backend Identifiers", but the table still lists display names like Postgres, Redis, MySQL, and SQLite. The new Selection contract only accepts the lowercase identifiers (postgres, redis, mysql, sqlite), so presenting the title-cased names here will mislead readers into using values that the code rejects.

## Backend Identifiers

| Backend    | Position                      | Typical Fit                                            |
| ---------- | ----------------------------- | ------------------------------------------------------ |
| Postgres   | Default production backend    | General-purpose production deployments                 |
| Redis      | Supported production backend  | Queue-centric, low-latency operational workloads       |
| MySQL      | Supported production backend  | SQL environments standardized on MySQL                 |
| SQLite     | Supported constrained backend | Embedded, single-node, or lightweight SQL deployments  |
| `InMemory` | Local/dev/test backend        | Examples, development, tests, and ephemeral evaluation |

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