Fully implement modern type hinting, checked with ty in CI

[Korijn]

Track C of the roadmap — the analog of observ #184 + #185, delivered as one PR instead of the planned stacked pair: the runtime changes typing needed turned out to be two trivial lines (an assert and a metadata fix), not enough to justify a standalone "runtime cleanups" PR.

Typing

• Operation TypedDicts in types.py — AddOperation, RemoveOperation, ReplaceOperation and the Operation union — used across the public signatures: diff() and produce() now advertise tuple[..., list[Operation], list[Operation]], apply/iapply take list[Operation], and to_json/to_str_paths are precisely typed.
• PEP 604 unions and builtin generics everywhere via from __future__ import annotations; the runtime floor stays Python 3.9 (typing-only imports live in a TYPE_CHECKING block, and nothing new evaluates at runtime).
• py.typed marker shipped in the wheel (verified in the built wheel) plus the Typing :: Typed classifier.
• Genuine annotation bugs fixed, as observ's pass also found: Pointer.evaluate claimed to return tuple[Diffable, ...] but parent is None at the root; PatchRecorder.record_add annotated reverse_path: Pointer with a None default.
• Duck-typed code stays honest: the iapply interpreter and diff()'s dispatch intentionally accept container look-alikes (observ proxies), so they unpack into Any/cast at one commented boundary instead of pretending to be nominally typed.

Toolchain

• ty in a dedicated ty dependency group (and in dev), [tool.ty] scoped to patchdiff/ with python-version = "3.9" so 3.9-incompatible typing constructs are caught by the checker.
• A dedicated Typecheck job in CI, installing only the ty group, added to publish's needs.

Runtime changes (annotation-driven, behavior-preserving)

• _Proxy._detach makes its caller-guarded invariant explicit with an assert.
• pyproject metadata fix: description was literally "MIT"; it's now a real description, plus license = "MIT" (the wheel now carries License-Expression: MIT).

Verification

• ty check: 26 diagnostics on master → 0 on this branch.
• Full test suite passes at 100% coverage on 3.10–3.14 locally (3.9 isn't installable in this environment; ty's 3.9 target and the CI matrix cover it).
• ruff check/format pass; the docs build stays warning-free with the new signatures rendered by mkdocstrings; built wheel inspected for py.typed and metadata.

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https://claude.ai/code/session_016Mu9vEBwU4fQLi8ZgkgdS2

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