docs(changelog): add changelog dir + Cobalt B20 Asset scheduled-multiplier entry

[robriks]

Summary

Sets up a per-hardfork, per-feature changelog pattern for base-std and lands the first entry.
Motivated by the chain.base.org vibenet demos work — we want
code-forward, per-line-item detail behind each hardfork changelog bullet (e.g. Cobalt's "Schedule
Multiplier Updates"), rather than a single prose blob.

• New changelog/ dir (top-level, dedicated). changelog/README.md codifies the standard:
  layout (changelog/<hardfork>/<product>-<feature>.md), what each entry must contain, the
  "verify every selector/topic0/interface-id against source" rule, and an index table.
• First entry: changelog/cobalt/b20-asset-scheduled-multiplier-updates.md — the Beryl → Cobalt
  ERC-8056 ("Scaled UI Amount") migration for the B20 Asset multiplier surface. Minimal and
  migration-focused: mapping table (functions/events/errors with real 4-byte selectors), the
  scheduled-update lifecycle, and a guarantees/edge-cases Q&A.
• Discoverability: linked from the top-level README.md.

The entry pairs with the interface rework in #192 (BOP-495) and the base/base AssetV2 (Cobalt)
surface. All cited selectors/topic0s were verified from src/interfaces/** and the frozen ABIs.

Layout

changelog/
  README.md
  cobalt/
    b20-asset-scheduled-multiplier-updates.md

Notes for reviewers

• Docs-only change — no Solidity touched; CI's forge build/test/coverage/fmt gates are unaffected.
• Proposing this dir layout + naming as the convention going forward (one file per scoped feature,
  product-prefixed, under a lowercase hardfork dir). Happy to adjust naming/placement — flagging
  @rayyan224 per the Slack thread.
• Cobalt is not live yet; the entry states this explicitly and every pre-existing Beryl selector
  remains callable.

Test plan

• Confirm changelog/ layout + naming is the pattern we want to standardize on.
• Verify links render (top-level README → changelog/README.md → the Cobalt entry).
• Spot-check a few selectors/topic0s in the mapping table against cast sig / the ABIs.

Made with Cursor

[robriks]

Superseded by #197, which originates from a branch on this repo (base/base-std) instead of a personal fork. Identical contents; closing this one to keep the branch on the canonical remote.