test(web): cover usdcToMicros and formatUsdcMicrosValue

[dirtybits]

Summary

Adds behavioral tests for the two USDC money-conversion helpers in
web/lib/agentvouchUsdc.ts:

• usdcToMicros(amountUsdc) — converts a human decimal USDC amount to the
  6-decimal micro-unit the app and on-chain contract use.
• formatUsdcMicrosValue(micros, minimumFractionDigits) — formats a micro
  value back to a human-readable USDC string.

These functions sit on every USDC amount a user sees and every micro amount the
app converts between the decimal world and the on-chain 6-decimal micro-unit,
so a subtle rounding bug here corrupts displayed balances, purchase amounts, and
the on-chain transfer amount.

Why

The only prior reference for these helpers was a source-text grep in
web/__tests__/components/wallet-detection-source.test.ts, which asserts the
import exists but says nothing about behavior. Per AGENTS.md §3.24, pure helpers
get real behavioral tests, not source-text assertions.

What was verified

• Value pins: every expected value below was confirmed against the live
  implementation with a throwaway probe run before the assertions were written,
  so the tests pin exact behavior, not just shape.
• usdcToMicros: 0 -> 0, 1 -> 1_000_000n, 0.01 -> 10_000n,
  0.1 -> 100_000n, 12345.6789 -> 12_345_678_900n, 0.0000005 -> 1n
  (halfway rounds up via Math.round), 0.0000004 -> 0n (below half truncates),
  1.23456789 -> 1_234_568n, sub-micro -> 0n, and rejection of negative and
  NaN/Infinity/-Infinity inputs with the clear message.
• formatUsdcMicrosValue: null/undefined -> "0.00", 0 -> "0.00",
  10_000n -> "0.01", 1_500_000n -> "1.50", 999_999n -> "0.999999",
  12_345_678n -> "12.345678", 1_000_000_000_000n -> "1,000,000.00",
  identical bigint/number/string input handling (1_000_000 / 1000000 ->
  "1.00"), and minimumFractionDigits overrides ("0.0100" for 4,
  "0.010000" for 6, "1.5" for 0).

Verification

• npx vitest run __tests__/lib/agentvouchUsdc.test.ts — 10 tests passed.
• Full local gate on this change:
  • npm run format:check — all files Prettier-clean.
  • npm run lint — passed.
  • npm run typecheck — passed (tsc --noEmit after next typegen).
  • npm run test — 153 test files / 1321 tests passed (web + cli workspaces).
  • npm exec --workspace @agentvouch/web -- next build --webpack — not
    runnable in this cron environment because web/.env.local / DATABASE_URL
    is absent; the build fails during prerendering on DATABASE_URL before it
    could compile the new file. This is a local-environment limitation (documented
    in AGENTS.md §1.3), not caused by this change — the change is a
    single, pure test file under web/__tests__/lib/ that no route or app
    module imports, so it cannot affect bundling or output.

Scope

One new file: web/__tests__/lib/agentvouchUsdc.test.ts. No source, config, or
dependency changes.

Rollback

Delete the new test file; nothing else was touched.

Notes

• Commit is unsigned in this headless cron environment (no local GPG key;
  git config user.signingkey is empty). This matches the precedent of prior
  cron PRs (chore(web): remove dead EIP-8004 feedback module #191, chore(web): remove dead author dispute helpers #192). If GPG signing is required, re-sign with:
  git commit --amend -S --no-edit && git push --force-with-lease.