fix(type-plus): keep ? and readonly of A in LeftJoin

[unional]

Fixes #205.

Problem

LeftJoin<{ a?: number; b?: string }, { b: boolean }> returned { a: number; b: boolean }. The general branch mapped A's non-colliding keys with { [k in Exclude<keyof A, keyof B>]: A[k] }. That key set is computed, so the mapped type is not homomorphic and drops ? and readonly.

Fix

{ [k in keyof A as k extends keyof B ? never : k]: A[k] }. A key-remapping as clause over keyof A keeps the mapped type homomorphic, so A's modifiers survive.

Collision rule: when both sides declare a key, B replaces the whole property, including its ? and readonly. This matches the existing rule that B wins on the type, and it is what the B half of the join already did. The TSDoc now states the rule.

The two short-circuits (IsEqual → A, IsDisjoint → A & B) already kept modifiers. They are now covered by spec tests, so all three branches are pinned to the same behavior.

Tests

• New spec cases in left_join.spec.ts: A's modifiers on keys B does not redeclare, B's modifiers, the collision case, and both short-circuits. The two new TSDoc @examples are pinned there.
• The new A-modifier cases fail against the old implementation and pass with the fix.
• test:type passes on TypeScript 5.4, 5.5, 5.6, 6.0 and 7.
• verify:pkg, biome check and docs:llms:check pass. Error snapshots are unchanged.

Changeset: patch

I chose patch. This is a defect fix: the output now matches what a left join is expected to produce, and other type-output fixes in this release line are also patches. The package is in the 8.0.0-beta pre-release, so a larger bump would not change the version line anyway. It can still break a consumer that relied on the wrong output, for example code that read a formerly optional property as required. If the Council wants that treated as breaking, change the bump to major.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 96934ef

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
type-plus	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a1fa278) to head (96934ef).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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  Files           43        43           
  Lines          231       231           
  Branches        51        51           
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  Hits           231       231           

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