fix(b20-stablecoin): hard-code decimals to 6 to eliminate zero-return window

[eric-ships]

BOP-342 BOP-349

Motivation

B20StablecoinToken::decimals() previously delegated to the storage adapter via TokenAccounting::decimals(), which returns 0 for an uninitialized slot. During the factory bootstrap window -- after set_code installs the marker bytecode but before the initCalls chain writes the token fields -- any external call to decimals() would return 0 instead of the correct stablecoin precision of 6. This is incorrect: stablecoin precision is a protocol-level constant, not a per-token configuration value.

What changed

• In B20StablecoinToken's dispatch, the decimals selector now returns B20Variant::Stablecoin.decimals() (always 6) instead of reading from the TokenAccounting storage adapter.
• A unit test is added verifying that decimals() returns 6 regardless of the value held in the backing accounting (including 0, which simulates the uninitialized-slot case).

What was tried / considered

The recommendation in the audit finding suggested adding a method override directly on B20StablecoinToken. Because the dispatch layer already handles per-variant logic and B20Variant::Stablecoin.decimals() already encodes this constant (used by the factory storage when emitting B20Created), routing through that existing constant keeps the source of truth in one place and avoids adding a new method to the trait hierarchy.

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[github-actions]

Review Summary

No issues found. The change is correct and well-scoped.

What was reviewed:

• crates/common/precompiles/src/b20_stablecoin/dispatch.rs — dispatch now returns B20Variant::Stablecoin.decimals() (constant 6) instead of reading from the storage adapter.

Assessment:

• The .expect() on line 133 is effectively infallible — B20Variant::Stablecoin.decimals() is a const fn that always returns Some(6). The expect message is descriptive. No runtime risk here.
• The fix correctly reuses the existing B20Variant::Stablecoin.decimals() as the single source of truth rather than introducing a new constant or method override, consistent with how the factory already uses this value.
• The test covers both the uninitialized-slot case (decimals = 0) and a wrong-value case (decimals = 18), confirming the dispatch always returns 6 regardless of storage state.
• The asset variant (b20_asset/dispatch.rs) correctly continues to read decimals from storage, since asset decimals are per-token — no consistency issue.

[github-actions]

✅ base-std fork tests: all 610 passed

base/base is fully in sync with the base-std spec.

[refcell]

LGTM