Add accessibility modifiers

[colemancda]

.accessibilityLabel, .accessibilityValue, .accessibilityHidden, .accessibilityAddTraits, and .accessibilityIdentifier — the last Tier-A system, and the one that was blocking shipping a real app.

Shape

These describe a view rather than changing it, so all of them fold into Compose semantics and nothing else: label → contentDescription, value → stateDescription, traits → Role.Button/Role.Image/heading()/selected, identifier → testTag. Hiding uses clearAndSetSemantics, which drops the node and its subtree — the right meaning for a container.

Verified against the real accessibility tree

Accessibility is unusually verifiable here: uiautomator reads the accessibility tree directly, so this is ground truth rather than pixel inspection.

• label — Text("7").accessibilityLabel("Unread messages") reports content-desc="Unread messages". The label replaces the underlying text, which is the whole point.
• hidden — the hidden view appears 0 times in the tree while its sibling appears once. It's genuinely gone, not just visually suppressed.
• traits — .isButton changes the node's class from android.widget.TextView to android.widget.Button; a neighbouring untagged Text stays a TextView as a control.

swift test covers emission for all of them (101 passing), including that accessibilityHidden(false) still emits — it has to be able to override an inherited hide — and that adding a description leaves the node type, children, and text untouched.

What I could not verify, and why

accessibilityValue (stateDescription), the .isHeader trait, and .accessibilityIdentifier (testTag) are all applied, but uiautomator's XML has no attribute for any of them — its node schema is a fixed set that excludes state description, heading, and test tags (the last needs testTagsAsResourceId). TalkBack and Compose UI tests read them; this harness can't. I'd rather flag that than imply I confirmed it.

Deliberately not included

.accessibilityElement(children:). I implemented it, and semantics(mergeDescendants = true) did not merge the children — the two texts stayed separate nodes in the tree. Rather than ship a modifier that silently does nothing, I removed it. (That failure mode is exactly the unread-detents bug from #58, so shipping it would have been the same mistake.) .accessibilityHint is also absent: Compose semantics has no hint equivalent, and faking it by appending to the label would corrupt what gets announced.