fix(kilnd,component): reject arg-arity mismatch instead of zero-filling + reporting wrong result as success (SR-53, closes #443)

[avrabe]

The bug (#443, #412 fabricated-reporting family)

kilnd invoked a param-taking export with zero-filled arguments when params couldn't be supplied and reported the wrong value as SUCCESS:

kilnd mod.wasm --function addone   →  ✓ returned I32(1)  (exit 0)   # addone(0), zero-filled
wasmtime run --invoke addone mod.wasm 5  →  6                        # correct

Second manifestation: a spec-invalid component whose canon lift declares a different param arity than its backing core function was ACCEPTED and run, though wasm-tools validate and wasmtime reject it. Only result arity was ever checked.

The fix (fail-loud on every surface)

• kiln-runtime stackless engine (execute_function_body locals init): removed the "pad with default values" zero-fill and silent truncation of extra args — arg count ≠ declared param count is now an Err (runtime_type_mismatch).
• kilnd execute_traditional_module: checks the export's declared param count before engine.execute and errors with function 'X' expects N argument(s) but none were supplied; kilnd cannot yet pass wasm function parameters. No CLI arg-passing flag added (kept minimal per the issue); zero-param entry points (_start, result-only exports) still run.
• kiln-component direct hosting: the canon lift's REAL param types are now resolved into DirectExportTarget.params and the export's registered FunctionSignature.params (previously always empty), so the existing validate_function_args arity check fires; call_direct_export also checks param arity alongside its existing result-arity check.
• Load-time validation: each lift's declared param arity is validated against the LIVE instantiated core module's function signature during from_parsed_internal — the mismatched component is rejected at load (exit 1), matching wasm-tools/wasmtime.

Tests (TDD, RED confirmed first)

• kilnd/tests/arg_arity_tests.rs: no-args on 1-param export → Err; _start/zero-param exports still run; engine-level — missing arg Err, extra arg Err, dbl(5) = 10 correct.
• kiln-component/tests/direct_export_arity_tests.rs: add with 0 args → Err; add(5,3) = 8 (this direction was ALSO broken before — correct args were rejected against the empty registered params); mismatched lift rejected at load.

Real-binary repro re-run: --function addone exits 1 with the actionable message; --invoke add pc.wasm (invalid lift) rejected at load, exit 1; valid component + correct args works.

rivet: SR-53 created (status: implemented, release v0.4.3). rivet validate: 0 errors (after rivet sync --local for the known stale gust cache).

Note: full cargo test -p kiln-component / -p kiln-runtime target sets have pre-existing bit rot on main (identical error counts on clean main, verified side by side); working invocation used: cargo test -p kiln-component --features std,kiln-execution --test direct_export_arity_tests plus the compilable maintained targets. No new clippy warnings (--no-deps counts identical to main for all three crates).

Closes #443

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https://claude.ai/code/session_01FcTUZgts331Z1TK3q8YBQj

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Metric	Base Branch	This PR	Change
Errors	0	0	0
Warnings	5	5	0

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