spike(codec): compose value conversion from shared semantic rules #343

Define conversion once for each logical value kind, then compose it through containers and scientific wrappers. Connect these rules to the callable contracts in #335.

The existing mapper collapses Python integers and floats to number. Return-schema generation and decoding make separate representation decisions. Reproduced outcomes are tracked in #331 and #332.

Scope:

• Specify logical value kinds, numeric constraints, wire encodings, and decoded JavaScript representations separately.
• Own one language-neutral value-contract description for policy, codec identifiers, and compatibility rules. Publish a frozen internal prototype interface for spike(codegen): compile complete callable contracts before emission #335.
• Keep Python encoding and TypeScript decoding implementations explicit. Generate shared constants and structural checks where useful.
• Apply primitive rules through records and collections, including supported scalar and model conversions.
• Make Torch reuse ndarray conversion by dtype. Preserve Arrow payload fast paths without adding a second full traversal for schema validation.
• Treat unsupported conversions as explicit outcomes, with paths and guidance.

Acceptance criteria:

• A bounded prototype covers a safe integer policy, Arrow float16 ndarray conversion reused by Torch, and nested records.
• The same integer policy feeds encoding constraints, generated types, and return validation. Keep the fix(codec): reject unsafe Python integers before JSON encoding #331 rejection policy unless a separately reviewed design changes it.
• Root and nested values apply the same rules; conversion shortcuts cannot bypass numeric checks.
• NumPy and Torch float16 return numeric values, including signed zero, subnormals, and finite extrema.
• Unsupported values, cycles, malformed envelopes, depth, and payload limits produce bounded failures.
• Bigint and dataclass extension sketches identify the rule and adapter changes needed for design(codec): define exact bigint transport and generated integer types #337 and design(codec): define validated dataclass value transport #339.
• Adding a value kind does not require unrelated policy branches across mapper, generator, validator, and codecs.
• Publish fixed proof fixtures and expected outcomes for downstream conformance in test(architecture): verify value contracts across generation and execution #344.
• Document wire compatibility, generated-wrapper regeneration requirements, and migration steps before implementation.

Limit the prototype to two engineering days, one design iteration, and an independent review. This bounds investigation; it is not an implementation estimate. Preserve the existing transports, IR 0.4 inputs, and value-RPC execution model. Broad bigint and dataclass implementation remain separate work.