refactor(discord): extract recovery backfill + reaction hooks into DiscordRecoveryBackfillMixin (adapter.py god-file slice R2)

[andrexibiza]

Summary

discord adapter.py god-file slice R2: extract the recovery-backfill + reaction-hooks clusters from plugins/platforms/discord/adapter.py into plugins/platforms/discord/recovery_backfill_mixin.py (class DiscordRecoveryBackfillMixin). Part of the repo-wide large-file decomposition (tracker #78647).

What changed and why

• RecoveryBackfill (2096–2745) + ReactionHooks (2942–2996) — 34 methods total — moved byte-verbatim into the new mixin (705 lines; diff vs original windows = 1 inserted blank only)
• Mixin-FIRST class line (hard requirement): class DiscordAdapter(DiscordRecoveryBackfillMixin, BasePlatformAdapter): — base on_processing_start/complete stubs @4925–4959 would otherwise silently kill reaction hooks
• C3 CommandSync (2747–2941) correctly preserved in place (BLOCKED-now per consensus — R1 callers @1864/2025/2054)
• 15-test file test_discord_missed_message_backfill.py stays green
• Double-blind: 2 analysts (fully converged) → consensus (R2-CONSENSUS.md) → implementer → commit lane verified verbatim → 3 seam-test bugs fixed (test-only) → 2 blind re-reviewers

Testing

• 59 passed (test_discord_missed_message_backfill.py + test_discord_recovery_backfill_seam.py)
• Seam test fixes were test-only (widened FakeReactionMessage.remove_reaction signature, added mentions param, relaxed sys.modules assert) — extraction untouched
• python -c "import plugins.platforms.discord.adapter" OK · git diff --check clean · LF-only · DCO signed

Coordination / interlock

• Epic: Part of Repo-wide godfile eradication: residual 2K tasks after #102117 #78647 · Target: Part of Shard plugins/platforms/discord/adapter.py (god-file decomposition) #78634
• KILL LOCK: part of the discord adapter kill record (posted on Shard plugins/platforms/discord/adapter.py (god-file decomposition) #78634) — mess (Extract the authorization cluster from DiscordAdapter (god-file decomposition) #75734, [Bug]: /handoff discord creates private thread with only the bot as member — destination user never added, thread invisible #67702, security(discord): bound image/animation/attachment HTTP response reads #60395, Bug: Discord reply_to_mode: false regresses to first on current runtime #29623), shard wave, 31 fixer PRs; indexed by large-file decomposition meta-issue Repo-wide godfile eradication: residual 2K tasks after #102117 #78647
• Sibling PRs: the 4 other discord slices — disjoint windows
• Dedup: no existing PR extracts recovery backfill (open-PR sweep clean)

Part of #78647
Part of #78634

Part of #79564
Part of #79772

[eyeonall]

Cross-PR integration note: #79651 moves lifecycle reaction methods into DiscordRecoveryBackfillMixin, while #89405 adds default inbound reaction-target recording in on_processing_start.

Proposed low-risk sequence:

1. Merge refactor(discord): extract recovery backfill + reaction hooks into DiscordRecoveryBackfillMixin (adapter.py god-file slice R2) #79651 as a pure extraction.
2. Rebase feat(discord): add plugin-owned reaction actions #89405 and carry self._record_inbound_reaction_target(event) into the extracted mixin's on_processing_start.
3. Run the focused reaction and recovery seam suites, including a regression where a plugin reaction action omits message_id and targets the inbound message.

#89405's manifest send path and raw-reaction dispatch can remain in adapter.py; they do not need to be folded into #79651. This preserves the feature ownership in #89405 and avoids mixin method resolution silently bypassing the default-target recording.

[andrexibiza]

The exact current head is 51e04cde0b91391ac572db43ab2475f99a3d44a5, with covered commit 51e04cde0b91. It reapplies the verified recovery-backfill extraction unchanged: the 34 recovery and reaction-hook methods are housed in DiscordRecoveryBackfillMixin, and the mixin is first in DiscordAdapter's bases so the base adapter stubs cannot shadow those hooks. The command-sync cluster stays in place, preserving its caller dependencies; the reapplication reflects that the adapter source remained unchanged across the stated base comparison.

For refactor(discord): extract recovery backfill + reaction hooks into DiscordRecoveryBackfillMixin (adapter.py god-file slice R2), GitHub reports check rollup SUCCESS on exact head 51e04cde0b91391ac572db43ab2475f99a3d44a5.