feat(ui): add intrinsic sheets and box semantics

[GenericJam]

Summary

• add intrinsic-content Sheet detents, including an optional maximum height
• validate and encode detents at the renderer boundary so raw/sigil nodes cannot bypass the contract
• measure, cap, re-clamp, and internally scroll intrinsic Sheet content on iOS
• expose generic Box accessibility labels, button roles, and disabled semantics
• include Sheet in both packaged platform tag manifests

Why this belongs in Mob

These are generic renderer capabilities used by composed UI libraries. A plugin can style and compose Mob.UI.sheet/2, but it cannot change how the core Sheet measures arbitrary Mob children or make the core Box renderer honor accessibility props without replacing those primitives with app-specific native leaves.

The coordinated generated-Android implementation is GenericJam/mob_new#39.

Verification

• mix format --check-formatted
• mix credo --strict
• mix compile --warnings-as-errors
• mix test --exclude macos_only --exclude requires_zig — 1,119 passed, 38 excluded
• git diff --check origin/master..HEAD

This PR intentionally does not bump or publish a package version. Release bookkeeping follows after upstream review.

[GenericJam]

Adversarial review addressed — 34866b5

Ran a high-effort adversarial pass over 894bf5d. Eight findings, plus two of my own. All fixed on this branch.

iOS Box — three interacting fixes

.allowsHitTesting(false) let taps fall through a disabled box. It makes a view transparent to touches rather than blocking them, so a Box with disabled: true used as a blocking overlay or dimmed backdrop passed taps straight through to the content it was meant to shield. Now .disabled(node.disabled), which blocks interaction in the subtree while still consuming the touch.

Toggling disabled destroyed the subtree's SwiftUI identity. ifLet is @ViewBuilder if/else — _ConditionalContent — so flipping the branch hands SwiftUI a structurally different view and tears the subtree down. A Box wrapping a TextField lost in-flight text and focus; one wrapping a Sheet re-presented it. Tap wiring now branches on handler presence only, with the disabled check inside the closure, and both traits are unconditional OptionSet modifiers (? .isButton : []).

Traits leaked to descendants without a label. .accessibilityAddTraits is only scoped when .accessibilityElement(children: .ignore) ran, which only happened inside the label branch — so a role-only box announced every nested Text as its own button. Collapsing now keys on label or explicit button role.

iOS Sheet

Content detents flashed a 1pt hairline on every presentation. Content can only be measured after the sheet is up, so the first detent came from sentinels (intrinsicContentHeight = 1, environment default 1) and resolved to .height(1) — on every MobSheetView, not just the first. Measurement is now Optional; unmeasured presents at .medium and switches to the exact height when the measurement lands. The environment default is 0 meaning "root geometry unknown", so it no longer doubles as a clamp.

The detent ignored the bottom safe-area inset. .height() sets total sheet height while the content region is inset by the home indicator, so an intrinsic sheet was ~34pt short on notched devices — last rows under the indicator, and scrollable when the content was meant to fit exactly. The preference now carries height and inset together.

The 0.9 ceiling is a named constant with its rationale.

Elixir

Validation messages regressed to one generic string with no got:. Restored per-rule messages — [:medium, :medium] says "must not contain duplicates, got: …" — plus a dedicated message for malformed %{type: :content} maps, which previously fell through to the built-in-subset branch and misdescribed the problem.

@external_resource added to both tag manifests in Mob.Sigil and Mob.ScreenCase. They're read into module attributes at compile time, so adding Sheet recompiled nothing for anyone with a warm _build — every path-dep consumer, i.e. the standard mob dev setup — leaving the sigil rejecting <Sheet> until a manual mix clean. This change is the first to actually trip it.

Documented two limitations the implementation implies: a scrollable child (scroll, lazy_list) reports full content height under .fixedSize, so it expands rather than scrolling independently; and invalid detents now raise at the renderer boundary rather than degrading to [:medium, :large].

On the native tests

They broke on four separate behaviour-preserving refactors here — naming a constant, unwrapping an Optional, adding the inset, making traits unconditional. They pinned source text, not behaviour. Retargeted at the mechanisms (the unmeasured fallback, the inset arithmetic, the non-branching traits, .disabled over .allowsHitTesting) so they break when behaviour regresses rather than when the code is tidied.

Verification

1119 tests, mix format, credo --strict, clang-format, swiftlint (only the pre-existing force_cast), and clang -fsyntax-only on mob_nif.m — all clean. Device verification of the sheet still to come.