feat(type-plus)!: merge Equal into IsEqual<$O>, migrate IsLiteral and Some (#707)

[unional]

Closes #707. Closes #334. Resolves #275: the filter form it asked for is IsNotEqual<A, B, { selection: 'filter' }>, which keeps A when it differs from B.

What changed

1. Equal merged into IsEqual. IsEqual<A, B, $O> now uses Equal's body and its namespace ($Options, $Default, $Branch, $Fn, $Same, $ToProps, _ExactEqual*). Equal is deleted. Its spec is folded into is_equal.spec.ts, and the function-parameter recursion now calls IsEqual itself. The commented-out alternate implementation was dropped. testType.equal still goes through IsEqual and is unchanged.

• $Options extends $Selection.Options (the full set, not the $BaseOptions Equal used), so { selection: 'filter' } works. Filter returns A.
• IsNotEqual<A, B, $O> mirrors it and gains $Branch and $Fn. NotEqual is removed.
• IdentityEqual is removed. It had more callers than the issue listed: is_object.ts and is_not_object.ts used it too. All callers now use IsEqual.$Same.
• Internal positional call sites moved onto $O: ArrayPlus.Filter, ArrayPlus.IndexAt, Some.StrictArray.

Behavior change (in the changeset): two distinct unique symbols now compare false, and so does a unique symbol against symbol. Two assertions flip: is_equal.spec.ts (testType.equal<typeof s, symbol>) and tuple.spec.ts:43 (['a'] & symbol vs symbol).

2. IsLiteral<T, $O> is wired like the #677 types: $Options, $Default and $Branch, branches resolved through $ResolveBranch, filter returns T.

2b. Some<A, Criteria, $O>. Decision on the open question: Mode moves into $O as { mode: 'strict' | 'loose' }, next to selection and the branch overrides. This is the consistent answer, and 8.0.0 has no compatibility window to protect. Some<A, C, 'strict'> becomes Some<A, C, { mode: 'strict' }>. Filter returns the whole array A. The internal helpers take a pre-resolved { $then; $else } object (Some._Branches). They recurse on Tail<A>, so the filter subject has to be fixed to the whole A before they run.

3. Removed CanAssign, StrictCanAssign, IsAssign, Extendable, NotExtendable, IsExtend and IsNotExtend. Extends.ts is deleted. CanAssign.ts now holds only the runtime canAssign(), whose types now use Assignable and an inline conditional. The internal call sites (PadStart, Some.LooseArray) use Assignable. Specs that used CanAssign/StrictCanAssign as a tool now use Assignable (with { distributive: false } where needed).

4. IsDisjoint gets a TSDoc note explaining that it deliberately takes no options. StringIncludes gets a note explaining why it stays positional.

Sweep

The multi-line sweep from the issue now returns only StringIncludes and NotUnknownOr, plus the known false positives ($Selection.Invert, $Distributive.Parse, $Exact.Parse, and the _*OrElse helpers).

Docs

Migration guide: the old deprecated-alias table is gone. Every removal now sits in the "what moved in 8.0.0" table, and the symbol paragraph is now a behavior-change note. Also updated: the website API pages (equality, testing, array, type-guards-and-assertions, type-functions), packages/type-plus/readme.md, src/array/readme.md, src/assertion/readme.md, and the regenerated llms.txt.

Error snapshot

Updated. The options_Equal_typo probe is replaced by typo and wrong-value probes for IsEqual, IsNotEqual, IsLiteral and Some. Some's mode union is declared 'loose' | 'strict' because TS 5.4 and TS 7 print 'strict' | 'loose' in different orders.

Verification

• pnpm verify:pkg: test:type on TS 5.4, 5.5, 5.6, 6.0 and 7, test:errors, build, verify:dts, coverage, size — all pass.
• pnpm docs:llms:check passes.
• biome passes on every tracked file.
• The full pnpm verify failed in my worktree only because biome picked up a local, untracked .agents/ file that CI never sees.

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: f1fef6d

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	Major

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3dc855f) to head (f1fef6d).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #709   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           43        43           
  Lines          231       231           
  Branches        51        51           
=========================================
  Hits           231       231           

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