Docs/b20 implementation

[soheimam]

What changed? Why?

Added implementation guides for b20 spec

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[cb-heimdall]

🟡 Heimdall Review Status

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[roethke]

Part of the #1766 split. One accuracy issue in the implementation samples.

[roethke]

Review — verified against current base/base-std (main)

Reviewed all four implementation guides; they're technically accurate and the examples compile/run as written. No blockers.

Confirmed against source:

• createB20(B20Variant, bytes32, bytes, bytes[]), getB20Address, isB20, isB20Initialized; B20FactoryLib.encodeAssetCreateParams/encodeGrantRole/encodeUpdateSupplyCap/encodeUpdatePolicy.
• IPolicyRegistry create/update calls, updatePolicy(bytes32, uint64), isAuthorized(uint64, address), PolicyType values, admin staging.
• The precompile testing flow matches LIVE_PRECOMPILE_TESTING.md (base-forge install, make fork-tests, LIVE_PRECOMPILES=true FOUNDRY_PROFILE=fork on port 8546).
• Seize scope naming (SEIZE_HOLDER_POLICY / SEIZE_RECEIVER_POLICY) matches current base-std; PausableFeature = 4 members incl SEIZE; ALL_FEATURES_PAUSED = 15.

Optional nits (non-blocking):

• deployment-and-initcalls-encoding.mdx: example uses pragma ^0.8.20 while base-std pins 0.8.30 (fine — interfaces target >=0.8.20 <0.9.0).
• policy-configuration-in-code.mdx: composite policies (UNION/INTERSECT) are fully implemented in the shipped IPolicyRegistry, but base-std's PolicyRegistry/README.md still calls them "planned for a future hardfork" — a one-line note that composite support may be activation-gated per chain would help. Also a redundant guard id == 0 || policyExists(id) (policyExists(0) already returns true for the sentinel).

Good to go from a correctness standpoint.