fix(schema): preserve morph input narrows in unions

[ShuviSchwarze]

Problem

When normalizing an unordered union, equal morph branches are merged into one morph. The current implementation rebuilds the merged morph's executable in validator from rawIn. rawIn intentionally strips transforms and predicates for schema algebra and introspection, so this can silently remove prior .narrow() validation.

This causes #1608: a piped branch narrow is no longer executed after the union is intersected with another piped/narrowed type.

Fix

Merge the branches' executable inner.in validators when both are available, preserving their full validation paths. Declared morphs intentionally may not have inner.in, so the existing rawIn merge remains as a fallback for that case.

Tests

• Adds a Pipe in Intersection after Union drops previous narrowing conditions #1608 regression: a narrowed piped union branch remains rejected after intersection.
• Adds coverage for unioning declared morphs, verifying the fallback does not attempt to access an absent inner.in.
• pnpm testTyped --skipTypes ark/schema/__tests__/morphs.test.ts ark/type/__tests__/narrow.test.ts (1,722 passing)

Fixes #1608

[pullfrog]

ℹ️ No critical issues — one formatting fix needed for CI.

Reviewed changes

• Preserve executable input validators when merging morph branches — ark/schema/roots/union.ts: the merged morph now uses matchingMorph.inner.in.rawOr(node.inner.in) when both branches have an inner.in, so narrows/predicates on the piped input survive the union merge; declared morphs (which may lack inner.in) still take the rawIn.rawOr fallback.
• #1608 regression test — ark/type/__tests__/narrow.test.ts: a piped + narrowed union branch remains rejected after the union is intersected with another piped/narrowed type. I confirmed this test fails against the pre-fix implementation (the assertion flips to false), so it genuinely pins the behavior.
• Declared-morph union coverage — ark/schema/__tests__/morphs.test.ts: unioning two declared morphs exercises the fallback branch without touching an absent inner.in.

The fallback condition is sound: a branch without inner.in is either declared or permissive, so falling back to the rawIn union does not drop a real narrow. pnpm testTyped --skipTypes is green (1,743 passing), and eslint is clean on the changed files.

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[pullfrog]

✅ No new issues found.

Reviewed changes (incremental)

• Prettier formatting of the #1608 regression test — ark/type/__tests__/narrow.test.ts: the PipedItem chain is now wrapped the way pnpm prettier formats it, so pnpm checkPrettier / pnpm prChecks pass. No behavioral change.

This is the only delta since the prior review (54aec7d → 2364060), and it resolves the single formatting finding from that review. The union-merge fix and the two tests are unchanged; pnpm prettier --check now reports all three changed files clean.

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