fix: three silent drops in aspect-key handling (._ depth, provides mask, quirk multi-def)

[sini]

Three defects that shared a shape: content was declared, accepted, and then never reached the output — with no error, no warning, and no diagnostic.

._ stopped synthesizing below the second nesting level

den.aspects.a.b._ resolved; den.aspects.a.b.c._ threw attribute '_' missing. aspectContentType synthesized _ only on the wrapper for its own key, and the two annotators below it had already diverged — the single-def path walked one level, the multi-def path recursed, and neither carried _.

Collapsed into one recursive annotateChildren: every node that gets provenance also gets its own ._, at any depth. Include lists now map over the annotated children rather than the raw ones, which is what the multi-def path's own comment required — raw children get anon-renamed per inclusion path and double-emit class content.

provides/_ remain structural keys, so ._ still never collects provides children at any level.

Four tests in include-children: depth 3, depth 4, depth-3 class-key exclusion, depth-3 multi-def.

A provides child masked a same-named class key

An aspect declaring provides.user alongside user-class content emitted no user class at all:

den.aspects.svc = {
  provides.user.nixos.services.openssh.enable = true;
  user.description = "pinguino";   # silently dropped
};

__providesForwarded listed every provides child so classification would skip the forwarded copies, but the forwarded value only reaches the top level when the aspect does not define that name itself — the merge is providesChildren // merged, so a direct key wins. Masking it anyway left the surviving value classified as neither class nor nested key.

Now reports only children the aspect does not define directly, at both the root-aspect and nested-wrapper sites. host-to-users reads the same marker to decide entity-named sub-aspect fan-out, so it now matches its own stated rule ("not from provides").

Quirk keys defined twice on one aspect collapsed to one empty value

An aspect defined across several files, each contributing the same quirk key, reached consumers as a single entry carrying none of the emitted data — one attrset whose only attribute was imports.

unwrapContentValuesList folds multiple definitions into { imports = vals; }. Right for a class key, where the module system merges them back into one value; wrong for a quirk key, where nothing merges and the wrapper is the value the consumer reads.

Split the two readings: unwrapContentValuesAll returns one value per definition, used by pipe emission in both emit-classes and the deferred-include drain in resolve.nix. Class emission keeps the collapsing form. Assembly needed no change — it already concatenates entries per scope and per pipe.

One test in pipes, stating the designed behavior (quirks accumulate). It sorts before comparing, since merge order is not a contract.

Verification

• nix develop -c just ci → 1084/1084, exit 0
• nix develop -c just fmt → clean
• just check minimal, just check default, just check example (each --override-input den .) → all checks passed