DAL: prepare skip-list cells to carry attestation lag (semantics & hash-preserving)

Overall Plan (attempt)

1. This MR (preparatory, no behavior change): extend DAL skip-list types/encodings so cells can carry the attestation lag used when they were produced, while preserving hashes for all legacy cells.
2. Enable alternate lag (internal): the ability to have a different lag than the legacy one, still not user-exposed.
3. Plug the ability to provide the lag from outside (i.e., when calling cell addition functions).

• (3-b) Adapt plugin if needed.

4. Adapt the refutation games' code.
5. Adapt the Rollup node.

Current stack of MRs:

For deps, see https://docs.google.com/spreadsheets/d/1rak6nBXnlNggnOnA5WTF7idHNXn0hsSCnVqrypHBew0/edit?usp=sharing

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What this MR does

Type extensions (backward compatible):

• Add attestation_lag_kind = Legacy and legacy_attestation_lag = 8.
• Introduce cell_id = { header_id : Header.id; attestation_lag : attestation_lag_kind }.
• Change skip-list cell content to store lag:
  • Unpublished of cell_id
  • Published of { header : Header.t; attestation_lag : attestation_lag_kind; ... }
• Provide helpers: attestation_lag_value, equality/pretty-printers, and content_id : cell_content -> cell_id.

Encoding strategy (hash-preserving):

• Keep existing legacy encodings (tags 2/3) and serialize exactly as before (no lag field in bytes), so pre-T cells keep identical hashes.
• On decode, legacy cells are decorated in memory with attestation_lag = Legacy; this decoration is not re-serialized in legacy cases.

API/Call-site updates (mechanical):

• Adjust modules and callers to go through Slots_history.content_id → cell_id.header_id when they need the slot id.
• Add a client-side safety check in dal_plugin_registration.ml: verify attested_level = published_level + attestation_lag_value for fetched cells; introduce error Attested_level_mismatch (no behavior change unless mismatch).

Semantics preserving:

• No change to ordering/search: still compare by (published_level, slot_index) for now.
• No change to attestation logic, proofs, or import rules.
• No new configuration exposed.