fix(ai-path): connect() onProgress no longer fires 'log' duplicates

[Sentinel-Bluebuilder]

Summary

connect({ onProgress }) used to fire both the structured stage event AND a sibling 'log' event for the same line, forcing every consumer to filter stage === 'log' to avoid triple-printing. This PR fixes the contract so onProgress only fires for documented stages.

• ai-path/connect.js: raw SDK logs now route to a new opts.onLog callback. They are NOT forwarded to opts.onProgress. JSDoc updated to make both callbacks explicit and to list the exact stages onProgress emits.
• ai-path/README.md: options table updated — onProgress, onLog, and silent: true documented separately.
• ai-path/FAILURES.md: API-CONTRACT entry added with migration note.

Why

From the x402 fresh-test/connect-from-live-docs.mjs consumer harness: every progress line printed three times. Tracing into ai-path/connect.js:509-511 showed the log: callback fanned out to opts.onProgress('log', msg) on top of the structured stage emit. Consumer had to add:

onProgress: (stage, msg) => { if (stage === 'log') return; ... },
silent: true,

…to make output usable. The documented contract was "one event per stage transition" — this fixes the implementation to match.

Consumer-app context

The friction was discovered while building x402's agent E2E test harness. The harness drives the full connect flow from the live published llms.txt docs. With the duplicated events, every progress line printed three times (raw SDK log + duplicated 'log' event + structured stage event). After consumer-side filtering the output became correct — but only because both ends knew the quirk. After this fix, the consumer just sets silent: true and reads onProgress cleanly.

Backward compatibility

• Consumers that already filtered stage === 'log' continue to work — they just never hit that branch anymore.
• Consumers that did NOT filter — and relied on the raw log spam — should switch to onLog. The behavior change is "fewer events," never "missing events."
• No public-API rename, no signature change to connect() beyond a new optional onLog field.

Test plan

• js-sdk lint/build clean (no test suite changes touched).
• x402 fresh-test harness (fresh-test/connect-from-live-docs.mjs) — once linked locally to this branch, output shows one event per stage with no 'log' siblings.
• After merge + npm publish: re-run the same x402 harness against the published blue-js-sdk@next and confirm clean output without consumer-side stage === 'log' filtering.