Make partial find scans resumable

[Widthdom]

Summary

A 250,000-line find --all regex scan exits partial with has_more:true but next_cursor:null, so the remaining matches cannot be enumerated.

Audit ID: R-08. Follow-up to closed #4578 and #4730's bounded scan/pagination contracts.

Expected behavior

Every partial scan returns a stable continuation bound to the query, scan mode, source identity, and generation.

PR scope

Keep this to find continuation for partial scans.

Implementation guide for Codex 5.6 sol xhigh

• Encode path/file ordinal, line/byte position, regex/literal/options fingerprint, and source/index generation.
• Resume at a record boundary without duplicate/missing matches, including multiline/Unicode/large-line cases.
• Return typed errors for stale/mismatched/malformed cursors.
• Test full page-walk, cancellation/timeout/byte limit, file changes, empty/final pages, and deterministic order.
• Update help/schema, run full tests, and add a bilingual changelog fragment.

Acceptance criteria

• The 250k-line corpus can be fully enumerated page by page.
• No gaps or duplicates occur.
• Invalidated cursors fail clearly rather than restarting.

[Widthdom]

Work started on branch fix-issue4863.