flowtest: five things the loader knows and reports late or never — an empty `expect:`, a wrong `workflow:` path, a signal scripted after the gate's timeout, a type mismatch printed as quoting, and an unknown field with no suggestion

Observed behavior

Each reproduced at eb8172f.

1. expect: {} passes. A case with no claim at all is green, and --fail-on-warning does not notice. The format's own principle (others: skipped exists so "adding a step fails loudly") does not apply to a case that asserts nothing.
2. A wrong workflow: path is a run-time case failure. workflow: ./no-such-workflow.yaml reports loading workflow ... no such file or directory as the case's failure, with no position and no did-you-mean, while every other name in the format (ran:, signals, steps) is refused at load with a line.
3. A signal scripted after the gate lapses gets no dedicated diagnostic. signals: [{name: deploy-approved, at: 48h}] against a gate with timeout: 24h produces three downstream failures; the transcript's t=24h0m5s approval waiting is the only clue. The loader holds both numbers.
4. A type mismatch reads as a quoting difference. expect.outputs: {iterations: "1"} against an int fails with expected "1", got 1. The quotes are the whole message.
5. An unknown field gets no suggestion. flow test on a file with outputs: where response: was meant answers unknown field "outputs" (the key is response, file.go:784; outputs is real only under expect:, file.go:998). run.go:1572,1601,1740 already produce did you mean %q? for step and signal names; field names, the first thing a newcomer gets wrong, get the bare refusal.

Desired outcome

Each is a load-time refusal or a load-time warning with a position:

• an expect: with no claim is refused unless the case says expect: {nothing: true} or the spelling the format prefers for "this case only proves the run completes";
• a workflow: that does not resolve is refused at load, positioned, with the nearest existing file suggested;
• a signal whose at: is past every gate it could answer is refused at load naming the gate and its timeout;
• output and input mismatches print expected string "1", got int 1 using the D1 type spelling;
• unknown keys get the same did-you-mean the names get, over the keys legal at that position.

Acceptance criteria

• Five fixtures under flowtest/testdata with # want lines (#1602's shape) pin each refusal.
• examples/ and flow init's scaffold pass unchanged.

Constraints and dependencies

• Adjacent: #1441, #1616, #1602, #1647 (type spelling in diagnostics).

Start by reading the loader and diagnostic paths referenced at file.go:784 and 998, then inspect the did-you-mean handling in run.go:1572, 1601, and 1740. Add five fixtures under flowtest/testdata following #1602's # want shape, covering the listed load-time refusals or warnings. Run the flowtest fixtures and verify examples/ and flow init's scaffold remain unchanged.