docs: how MEV capture fees are credited (LP disclosure)

[moodysalem]

Adds an LP-facing note on how MEV capture's additional fee is credited, using the CSO-cleared text (EKU-963, verdict CLEARED WITH EDITS) word for word.

Changes

• concepts/extensions.md: new ### How MEV capture fees are credited section directly below the extensions table. The body is the CSO final text; its bold lead-in became the heading so the anchor can be linked.
• products/liquidity.md: the MEV capture bullet now ends with the CSO's one-line pointer, linking to /concepts/extensions/#how-mev-capture-fees-are-credited.
• Not changed: the table row, reference/contracts/evm-v3.md, and the whitepaper.

Checked against the code (evm-contracts bbda78f, the deployed runtime of 0x5555fF9F…41Be): in MEVCapture.accumulatePoolFees and the forwarded swap path, the held fees are only credited when lastUpdateTime != block.timestamp, through CORE.accumulateAsFees. That function credits the current active liquidity and burns the amount when liquidity is 0.

Verification: bun run build passed, bun run check:links passed (49 pages), Prettier is clean, and the anchor id="how-mev-capture-fees-are-credited" and the link from the liquidity page are both in the built HTML.

As the CSO asked, do not add redirect/JIT recipes, example numbers, or step sequences in later edits. Routine review, merge and normal existing-CI release are owned by CTO under EKU-1557; the obsolete blanket board merge card is withdrawn. No new deployment mechanism or feature-launch authorization is implied.

Feature lineage and acceptance criteria

Originating board feature: EKU-946, under EKU-928. Delivery: EKU-964, via EKU-960 and the decision in EKU-961.

User outcome: existing MEV capture LPs can understand when and to whom additional fees are credited.

Acceptance: CSO-cleared paragraph unchanged below the extensions table, working pointer from the liquidity bullet, excluded reference/whitepaper/table content untouched, checks pass, CTO decision recorded, and section/link verified on docs.ekubo.org after normal CI publication.

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