Add changelog entry

Co-authored-by: Agustin Mista <agustin.mista@moduscreate.com>
Co-authored-by: Alexander Esgen <alexander.esgen@iohk.io>
Co-authored-by: Georgy Lukyanov <georgy.lukyanov@iohk.io>
Co-authored-by: Thomas BAGREL <thomas.bagrel@tweag.io>
Co-authored-by: Nicolas BACQUEY <nicolas.bacquey@tweag.io>
Co-authored-by: Nicolas "Niols" Jeannerod <nicolas.jeannerod@moduscreate.com>

Author: 6 people

ouroboros-consensus/changelog.d/20250918_114333_thomas.bagrel_hfc_era_peras.md

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+<!--
+A new scriv changelog fragment.
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+Uncomment the section that is right (remove the HTML comment wrapper).
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+
+<!--
+### Patch
+
+- A bullet item for the Patch category.
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+-->
+<!--
+### Non-Breaking
+
+- A bullet item for the Non-Breaking category.
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+-->
+
+### Breaking
+
+- Changes in the HFC types:
+  - `EraParams` now keeps track of an optional Peras round length.
+  - `Bound` now keeps track of an optional Peras round number.
+  - In the `Serialise` instances for `EraParams` and `Bound`, the encoders generate different CBOR depending on whether of not the Peras-relate components are present. The decoders act differently depending on the length of the CBOR list.
+- Changes in the HFC time translation queries:
+  - Two new top-level queries are exposed from `Ouroboros.Consensus.HardFork.History.Qry`:
+
+    ```haskell
+    perasRoundNoToSlot :: PerasRoundNo -> Qry SlotNo
+    slotToPerasRoundNo :: SlotNo -> Qry PerasRoundNo
+    ```
+
+- Add a roundtrip test that ensures that converting Peras round number to a slot and then back is an identity is added into the `Test.Consensus.HardFork.Summary` module.
+- Add a Peras-specific test into `Test.Consensus.HardFork.History` module.
+
+### Non-Breaking
+
+- The `EraSummary`, while not modified directly, is now Peras-aware via `EraParams` and `Bound`:
+  - in a valid summary, Peras round length must divide the epoch size. See the changes in `EraSummary`s haddock and in `invariantSummary`'s implementation.