docs(b20): align Tokenize Assets and Stablecoin guides with base-std guides

[soheimam]

What changed? Why?

Aligns the Build on Base B20 guides (Tokenize Assets, Issue Stablecoins) with the guides in base/base-std/docs/guides. Principle: Build on Base keeps its page shape (intro, Vibenet demo, verified TS/Solidity/CLI code group, See Also); base-std supplies the content (Before You Start, steps, verify signal, check-order error tables). Language follows the Legal B20 guidance doc: asset-agnostic framing, stock token as one worked example, no "security"/"equity" phrasing, multiplier framing retained.

Pages

• Recover Funds: rewritten around seizeWithMemo (the base-std path) instead of deprecated burnBlocked + mint. totalSupply unchanged, Seized event, memo for case reference.
• Cancel Blocked Units → Seize and Cancel Units: seize to treasury, then burnWithMemo. Redirect added.
• Announce a Distribution → Announce a Corporate Action: asset-agnostic intro, base-std four scenarios (additional issuance, multiplier update, treasury burn, notice). Dividend remains the worked example. Redirect added.
• Apply a Multiplier: title kept per Legal; adds Before You Start, the one-pending-update lifecycle, effectiveAt() > block.timestamp rule, pause guidance as a note.
• New: Restrict Who Can Initiate Transfers (TRANSFER_EXECUTOR_POLICY), transfer agent as one example. No demo flow exists for it.
• Block an Account: new section on attaching the blocklist to SEIZE_EXEMPT_POLICY so a hold is also recoverable.
• Cobalt warning on both seize pages (live on Vibenet, mainnet after the hard fork).

Correctness

• SEIZE_HOLDER_POLICY → SEIZE_EXEMPT_POLICY (post-Cobalt name) in concepts/policies.mdx, reference/constants.mdx, reference/errors.mdx, with the correct hash. Note: the upstream base-std seize guide still uses the old name.

Demos (only the two flows whose pages changed)

• Vibenet engine v4: seize helper, SEIZE_* scopes, SEIZE_ROLE, AccountNotSeizable/InvalidReceiver/InvalidSender decoding.
• Asset cancel flow: Fund → Block → Seize → Cancel. Stablecoin recover flow: Setup → Seize → Reissue. Live and offline mock paths both updated. All other flows untouched.

Samples and tooling

• New verified fixtures stock-seize-*, stock-executor-*; stablecoin-recover-* rewritten. stock-cancel-* removed.
• base-std pin moved from be6d045 to 1505323 in verify-doc-samples.sh and page notes (the old pin does not contain seizeWithMemo; the previously vendored lib did not compile the existing multiplier sample either).
• Nav, redirects, base-std sync route-table.json (renamed pages plus a rule for restricting-transfer-initiators.md) and its test updated.

Notes to reviewers

• Legal: titles and prose were checked against the B20 language guidance. "Schedule a Stock Split" was deliberately not adopted as a title; the multiplier framing stays. "Corporate action" is used only with asset-agnostic context and multiple examples, per that guidance.
• The Asset "cancel" and Stablecoin "recover" demo flows should be clicked through on the preview; they compile and the engine unit test passes, but I could not exercise the live path headlessly. A live Vibenet B20 token was confirmed to answer SEIZE_ROLE() and SEIZE_EXEMPT_POLICY().
• SEIZE_ROLE is now granted at demo token creation for all flows (one extra role grant); happy to narrow it to the two seize flows if preferred.
• Live-evidence hashes in verification-manifest.json still reference the earlier Base Sepolia runs; regenerating needs a funded key.
• One pre-existing failure in base-std-routing.test.mjs (errors.md → events.mdx expectation) exists on master and is unrelated.

How has it been tested?

• node scripts/verify-doc-samples.js: 73 samples in sync.
• npx tsc --noEmit (typescript samples) and forge build (solidity samples, base-std@1505323): clean.
• node scripts/test-vibenet-engine.mjs: pass.
• npm test: 85 pass, 0 fail.
• JSX snippets compiled with esbuild; every internal link and nav entry in the changed sections resolves to a file.

Screenshots

Pending: preview build of the changed pages.

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