forms: emit x-submitMode from the action type

[Yaraslaut]

Closes #208.

The gap

x-submitMode was documented in the spec and implemented in DynamicForm.qml,
but nothing in C++ emitted it — so no schema generated from a compiled
action type could carry it. Ten call sites across polls, pastebin and
bookmarks hand-write the same workaround instead (DynamicForm { controller: null } plus an external Button), with comments naming each other as the source
of the copy.

The spec sentence declining to emit it refuted itself: it said an author sets the
key by hand "the same way x-layout/x-widget overrides are authored today" —
but both of those are emitted from C++ declarations, and x-layout is a
top-level key emitted from a static constexpr formLayout, structurally exactly
what this needed. The mechanism already existed, twice.

What changed

struct CreatePaste {
    std::string title;
    std::string body;
    static constexpr bool explicitSubmit = true;
};

schemaJson<A>() now emits the top-level "x-submitMode": "explicit", following
formLayout's pattern.

The design question, and how I resolved it

#208 leaves opt-in trait vs. derived-by-default open. I took opt-in, and
this is the one judgement call in the change, so it is worth stating plainly:

• Deriving from a "does this action mutate" predicate would need a predicate
  forms.hpp does not have, and would flip the rendering of every existing
  generated form at once — a shipped-behavior change, not an emitter addition.
• Opt-in matches how formLayout, fieldSpans and formRules already work,
  keeps the decision with the author who knows whether the action has effects,
  and changes no existing schema: an action declaring nothing, or declaring
  false, emits no key.

Derived-by-default remains available later; this makes the feature reachable
without committing to it. If you'd rather have the derived version, this is cheap
to revisit.

Deliberately out of scope

Migrating the ten existing call sites off the workaround. That alters three
shipped apps' behaviour and belongs on its own change, not bundled with the
emitter.

Verification

The issue flags invariant 7 explicitly, and it is the crux here: today's only
coverage is a hand-written schema literal (tst_DynamicFormSubmitMode.qml),
which keeps passing with the emitter removed entirely. So the new fixtures
are driven through schemaJson<A>().

Mutation-checked — disabling the emitter fails 2 of the 4 new cases,
while the two "omits the key" cases correctly still pass:

CHECK( schema.find(R"("x-submitMode":"explicit")") != std::string::npos )
test cases: 4 | 2 passed | 2 failed

One case parses the schema rather than substring-matching, so a key that landed
inside properties instead of at the root fails there.

check	result
morph_tests	20630 assertions in 1158 cases, all passed
new [submitmode] cases	8 assertions in 4 cases
compiler warnings	0 (initial draft used the deprecated glz::json_t; now glz::generic)
Doxygen (WARN_AS_ERROR)	clean
scripts/check_spec_citations.sh	pass

Note on the test fixtures

They are file-scope rather than anonymous-namespaced, deliberately: glaze's
reflection takes the address of an extern const T, which a type with no linkage
cannot have — an anonymous-namespace action fails to compile. The SM prefix
keeps them unique for the file-scope-collision CI check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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