feat: infer query/command argument types from the handler signature

[SomaticIT]

What

Allows declaring query, query.batch, query.live and command with just a handler — the client-side callable gets the exact same signature as the handler's parameter, type-checked by TypeScript, with no runtime validation:

const my_query = query(({ filter, sort }: { filter?: string; sort?: string }) => ...);
my_query({ filter: 'name eq Claude' }); // type-checked against the handler's parameter

SvelteKit can't offer this because remote-function arguments cross a network trust boundary; locally the handler and caller live in the same bundle, so the handler's signature is authoritative.

Semantics

• One-argument rule kept: exactly one argument, still used as the stable-JSON cache key (same dedup rules).
• Runtime behavior is passthrough — equivalent to 'unchecked'. Schema and 'unchecked' declarations are unchanged (kit parity preserved).
• form is excluded: form data comes from the DOM as strings and needs coercion + validation, so forms still require a schema or 'unchecked'.
• Bare-handler arity is detected via fn.length: zero-parameter handlers stay argument-less (the dev-time guard against accidental arguments is kept). Documented caveat: handlers with only default/rest parameters ((arg = {}) => ...) have length 0 and are treated as argument-less.

Implementation

• parse_declaration() in src/lib/validation.ts is the single choke point all factories flow through: a bare handler with fn.length >= 1 now maps to 'unchecked'. This activates the feature for query, command and query.live with no factory changes.
• query.batch additionally accepts the bare-handler form (create_query_batch now uses parse_declaration; a batch handler always takes the args array, so a bare declaration simply means unvalidated).
• New TypeScript overloads on query / query.batch / query.live (in src/lib/query/index.ts) and command (in src/lib/command.svelte.ts), placed after the existing no-argument overloads so zero-param handlers keep resolving to LocalQueryFunction<void, Output>.

Docs & tests

• New DIFFERENCES.md section "Validation is optional for query and command" with rationale and the arity caveat; README examples updated; playground switched to the inferred form for getLikes/addLike.
• 8 new tests (103 total): inferred args for all four factories, passthrough (no coercion), cache-key dedup, dev-guard regression for default-parameter handlers, and @ts-expect-error compile-time assertions.

Verification

• svelte-check: 0 errors / 0 warnings · vitest: 103/103 · svelte-package + publint: clean
• Playground driven by Playwright/Chromium: 12/12 checks with the inferred-form declarations

🤖 Generated with Claude Code

https://claude.ai/code/session_01EJj9ab3v9HjudgX8L1x821

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