feat(type-plus): add .$Fn to the two-input predicates

[unional]

Part 3 of #701, the last part. Parts 1 (#703) and 2 (#704) are merged.

The parameter order follows the decision on the issue's open question: the function's input is the value being checked, and the other input is fixed as .$Fn's first type parameter, ahead of $O.

type R = TuplePlus.Filter<[1, number, 1], Equal.$Fn<1>> // [1, 1]
type R = TuplePlus.Find<[string, 1], Assignable.$Fn<number>> // 1
type R = TuplePlus.Filter<[{ a: 1 }, {}], HasKey.$Fn<'a'>> // [{ a: 1 }]

Changes

• Added .$Fn<Fixed, $O> to Assignable, NotAssignable, Equal, HasKey and IsOptionalKey. $O is checked by $StrictOptions, as on the other .$Fns.
• Each has a TSDoc example, pinned in that symbol's own spec.
• Updated guides/type-functions.mdx with the shape and the reason only one of the two inputs can be the entry.
• New Filter benches for Equal.$Fn, Assignable.$Fn and HasKey.$Fn.
• Added a minor changeset.

No reverse-direction variant ($FnFrom), as decided. It can follow if a use case appears.

Instantiation cost

Per TuplePlus.Filter call on a distinct 10-entry tuple, 100 uses:

Type	TS 6.0	TS 7
Filter+object	679.7	679.7
Filter+IsObject.$Fn	967.3	967.3
Filter+Equal.$Fn	952.2	952.9
Filter+Assignable.$Fn	1112.0	1112.7
Filter+HasKey.$Fn	857.3	858.3

Equal.$Fn<1> costs about the same as IsObject.$Fn, so an exact match is no more expensive than any other predicate function.

Verification

• Type tests pass on all five compilers: pnpm --filter type-plus test:type.
• test:errors matches the snapshot on all five compilers.
• pnpm verify passes. Locally, check flagged only an untracked harness file that is not part of this PR.

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: cbab0f0

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	Minor

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ab2f472) to head (cbab0f0).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           43        43           
  Lines          231       231           
  Branches        51        51           
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  Hits           231       231           

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