docs: reduce the README banner to the brand lock-up - #143

Summary

Follow-up to #140, which merged before this correction existed.

#140 put the whole .top-nav strip into the README: the network badge, token
badge, theme control and workspace link travelled with it, none of which mean
anything outside the running app, and at README width the full-width row
rendered small. This reduces the banner to the brand lock-up alone — the
commit-ring mark, the wordmark, and its SETTLEMENT ENGINE tag — on the same
rounded panel.

Every length is now the corresponding .top-nav value scaled by one factor
(64 / 36, the mark's banner size over its nav size), so the proportions stay
exactly those of the shipped nav while the lock-up reads at README width. The
canvas is 381x112 and sized to its own content rather than fixed.

The banner is still generated, never drawn: the palette comes from
packages/brand/src/tokens.css, the mark geometry from
packages/brand/src/CommitRing.tsx, and both labels from
apps/web/src/App.tsx. tokens.css remains the only source of a brand colour.

Scope and acceptance criteria

• The change is limited to the stated milestone or issue.
• Acceptance criteria are listed and satisfied.
• No unrelated cleanup is included.

Acceptance criteria:

• The README banner shows the brand lock-up only, at a size that reads in a
  README column.
• Proportions remain those of .top-nav; no length is retuned by hand.
• No colour, mark geometry, or label is retyped; the renderer still reads all
  three from front-end source.
• The tag clears the panel edge with the fallback fonts GitHub renders.
• markdownlint, prettier --check, and eslint stay green.

Product and security invariants

• Tenant isolation remains fail-closed.
• Sponsor authorization, auditability, and daily caps remain enforced where applicable.
• Recipients cannot modify sponsor controls or access sponsor-only data.
• No secret, token, production identifier, or personal data is committed or pasted into review prompts.
• Any non-applicable invariant is explained below.

Invariant notes:

Documentation and one build-time renderer only. No runtime code, contract,
migration, or configuration is touched, so the isolation, authorization, and
cap invariants are unchanged by construction.

Validation

Commands and results:

node scripts/render-nav-panel.mjs: PASS (wrote docs/assets/nav-panel-{light,dark}.svg, 381x112)
text extent measured in-browser with fallback fonts: PASS (tag ends at x=348 in a 381 canvas; 33px right padding against 35.6 left)
npx markdownlint-cli2@0.18.1 README.md: PASS (0 errors)
prettier --check scripts/render-nav-panel.mjs: PASS
eslint scripts/render-nav-panel.mjs: PASS

Independent review evidence

Gate A — exact candidate tree before push

• Base commit SHA: 9177bbee4aa3705eae2038877f8183614ca89364

• Reviewer tool: not run

• Verdict: not run

• Findings or residual risks: Gate A was waived by the user for the README work
  in docs: refresh README and add front-end derived banner #140; this follow-up carries the same waiver.

• The reviewed tree equals the committed tree.

Gate B — exact remote PR head

• Pull request URL/number: this PR

• Remote head commit SHA: 5fd1d57

• Reviewer tool: not run

• Verdict: not run

• Findings or residual risks: waived with Gate A.

• Gate B reviewed the current remote head and matches Gate A's approved tree, or a fresh Gate A was run for the changed tree.

• Agent policy / repository-policy and all applicable CI checks pass.

Risk and rollback

• Residual risks: no independent review evidence backs this tree. The SVGs are
  static build output, so a brand token, mark, or label change needs
  scripts/render-nav-panel.mjs re-run or the README drifts from the UI again.
  Pill and panel widths come from per-family advance estimates, since Rubik
  cannot be measured without a font engine.
• Rollback or recovery plan: revert the single commit; the README returns to
  the wide strip merged in docs: refresh README and add front-end derived banner #140 and no runtime behaviour changes.

Human merge

• A human owner has reviewed the evidence and will perform the merge.

Draft: opened as a draft with Gate A and Gate B waived, matching how #140
was opened.