feat(binding): support composite capability outputs

[jmgilman]

Summary

• compile nested output type graphs into immutable reflection plans at registration
• convert structs, arrays, slices, string-key maps, pointers, named scalars, integers, floats, and bytes into bounded process-neutral values
• reject dynamic, custom-marshaled, cyclic, and otherwise unsupportable graphs before serving
• derive deterministic flat nested output notation without changing the MCP schema
• thread remaining aggregate result bytes into parent conversion preflight while leaving exact debit at the worker frame boundary

Part of #23. The real MCP composition proof and documentation follow in later increments.

Resource and safety behavior

• preflight container lengths before proportional maps, lists, or sorted-key allocation
• preserve nil versus empty and optional omission versus required nullable fields
• reject uint overflow and non-finite floats as capability failures
• classify depth, node, per-value, and remaining aggregate exhaustion as resource limits
• keep handlers and output reflection in the parent; only process-neutral values cross to child Starlark

Validation

• go test ./internal/binding ./internal/worker .
• golangci-lint run --config .golangci.yml ./internal/binding ./internal/worker . --show-stats=false

Review

Bounded persistent review verified allocation ordering, type-cycle rejection, nil/omitempty semantics, numeric safety, deterministic paths, worker-boundary preservation, budget threading, and caller migration. Its named-byte-marshaler rejection finding was fixed with slice and array regressions; its eager conversion-state allocation observation was also removed.