v8.5.6: dm experimentally supports foreign key (#21523)

[ti-chi-bot]

This is an automated cherry-pick of #21523

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

• Update four docs pages to document DM's experimental foreign key support shipping in v8.5.6
• Covers three engineering PRs: tiflow#12351 (safe mode FK), tiflow#12414 (multi-worker causality), tiflow#12329 (DDL whitelist)
• All behaviors validated against a live build (v8.5.5-12-gd6d53adbe) with 13 test scenarios, all passing

File	Change
dm/dm-safe-mode.md	Add "Foreign key handling (experimental)" section: non-key UPDATE optimization, session-level FK_CHECKS toggle, multi-worker causality. Add forward-reference from Working principle.
dm/dm-compatibility-catalog.md	Rename "Incompatibility with FK CASCADE" to "FK CASCADE operations". Add experimental support with validated constraints (PK/UK guardrail, DDL rejection, routing, BAL ancestor, ON UPDATE CASCADE mismatch).
dm/dm-precheck.md	Update FK warning: TiDB supports FKs (GA since v8.5.0), DM experimental since v8.5.6.
foreign-key.md	Replace "DM does not support foreign keys" with experimental v8.5.6 support, warning, and backward-compat note.

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What is the related PR or file link(s)?

• This PR is translated from: v8.5.6: dm experimentally supports foreign key docs#22616
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
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/approve

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