fix: surface union branch errors instead of discarding them

[1222hxy]

Problem

The union resolver collects each branch's ValidationError into messages, then throws without ever reading it:

schemastery/packages/core/src/index.ts

Lines 704 to 714 in cf0b7e5

	Schema.extend('union', (data, { list, toString }, options, strict) => {
	const messages: any[] = []
	for (const inner of list!) {
	try {
	return Schema.resolve(data, inner, options, strict)
	} catch (error) {
	messages.push(error)
	}
	}
	throw new ValidationError(`expected ${toString()} but got ${JSON.stringify(data)}`, options)
	})

const messages: any[] = []
for (const inner of list!) {
  try {
    return Schema.resolve(data, inner, options, strict)
  } catch (error) {
    messages.push(error)     // never read
  }
}
throw new ValidationError(`expected ${toString()} but got ${JSON.stringify(data)}`, options)

messages is write-only. Linters do not flag it because push counts as a use.

For a discriminated union this discards exactly the information the caller needs. Given:

const config = Schema.union([
  Schema.object({ a: Schema.const('foo'), b: Schema.number().required() }),
  Schema.object({ a: Schema.const('bar'), c: Schema.string().required() }),
])

config({ a: 'foo' })

Before — every branch's full signature, but not the one thing that is wrong:

expected { a: "foo", b: number } | { a: "bar", c: string } but got {"a":"foo"}

After:

expected { a: "foo", b: number } | { a: "bar", c: string } but got {"a":"foo"}
  - $.b missing required value
  - $.a expected bar but got foo

Both detail lines were already computed one line earlier and thrown away.

Why only branches that failed deeper

A branch that failed at the union's own path only restates the mismatch the top-level line already reports, so reporting it is pure noise. A branch that failed at a deeper path names the offending property, which the caller cannot recover otherwise.

Filtering on path depth keeps the change free for the cases that gain nothing:

Schema.union([1, 2])('3')
// expected 1 | 2 but got "3"        (unchanged — both branches failed at the union's own path)

Schema.union([Schema.object({ a: Schema.number() }), Schema.number()])('foo')
// expected { a?: number } | number but got "foo"    (unchanged)

Absolute paths are preserved through nesting, so the detail stays actionable inside objects and arrays:

$.x expected ... but got {"a":"foo"}
  - $.x.b missing required value

$[0] expected ... but got {"a":"foo"}
  - $[0].b missing required value

Duplicate messages are collapsed.

Scope

Only the message of an already-thrown error changes. Successful validation, the set of inputs that throw, and the error type are all untouched.

Tests

Added error detail to packages/core/tests/union.spec.ts covering the surfaced property errors, absolute paths under nesting, and the unchanged primitive-union message.

yarn yakumo test core goes from 45 to 46 passing with no new failures.

Note on CI

main is currently red independently of this change, so a red check here is expected:

• packages/core/tests/i18n.spec.ts fails on main (it outlived cf0b7e5 "refa: remove i18n dependency").
• The test script passes -r esbuild-register, which current yakumo rejects with unknown option: "r".

I left both alone to keep this PR to one concern, but happy to fix either here or separately if useful.