refactor(pagination): single Paged representation across β/γ/local (fixes defect 1 & 5)

[allmonday]

Summary

Lands the five-step plan from the full-project orthogonality review (mindmap node #14 / 收敛路线⑤ pagination half): the four pagination numbers (limit / offset / order / direction) used to change shape ~10 times across 3 transport mechanisms, with 2 merge implementations and a double read of the same raw args. This PR makes the merged Paged the single representation end to end — and fixes two of the five confirmed defects on the way.

The five steps

#	Change	Effect
①	paged_from_selection() — the single dispatch-layer read of raw selection.arguments; the paginated loader's in-loader GAP B fallback re-parse is deleted	Same intent parsed once, one layer
②	γ loader's create-time closure defaults (order/direction/limit/offset — dead config, the only caller never passed them) and their eff_* merge chain deleted	One merge implementation (Resolver._merge_paged)
③	paged_wire_params() — single Paged → wire-fields mapping; β gql renderer and γ JSON body both consume it	Adding a future param (e.g. cursor) = one field + per-codec formatting
④	Page params join the loader split key via the existing params_key mechanism	Fixes defect 1 (see below)
⑤	Paged.clamp() — single clamping implementation, applied at the β dispatch layer against the relationship's max_page_size	Fixes defect 5 (see below)

Defect 1 — concurrent param overwrite (data correctness)

REMOTE_PAGED selections are wrapped in {items, pagination} — neither key is a target-entity field, so the selection fingerprint (type_key) was always None there and force_split never fired: concurrent reviews(limit:5) / reviews(limit:10) loads shared one loader instance and overwrote each other's _remote_page_params. Params now join the split key (the same per-params mechanism γ already used); identical params still share an instance so batching is preserved.

Defect 5 — federation limits forwarded unclamped

max_page_size clamping existed only on the local PageArgs path; β/γ forwarded limits as-is (limit=100000 vs a declared max of 100 went straight to the member). Paged.clamp() is now the single implementation, applied at the β dispatch layer. limit=None (remote full-fetch) semantics unchanged. γ has no boundary declaration surface today (RelationshipInfo is a β concept) — noted in-code as a follow-up decision.

API change

create_dto_remote_loader drops its four dead pagination kwargs — never effective in-tree (the only caller never passed them); its only direct test is updated to the side-channel contract, which also locks the explicit limit=0 vs omitted distinction.

Verification

• New regressions: params_key isolation + the type_key is None root-cause lock, wire clamp (100000 → 100), γ full-fetch back-compat, loader-never-rereads-args contract
• Full suite 1683 passed / 0 failed; ruff check src/ clean

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