fix(correlation): resolve bare tickers so the matrix stops failing with "Fetched: []"

[yxhuang]

Summary

Fixes the Correlation tab failing with Could not fetch price data for at least 2 assets. Fetched: [] when given bare tickers like AAPL,SPY — the exact format the UI hint suggests. Root cause: user input was passed straight to loaders that key instruments by exchange-suffixed symbols (AAPL.US / 600000.SH), plus two compounding issues in market inference and the fetch loop.

What's in it

Three layered fixes in agent/backtest/correlation.py:

• infer_market rewritten — explicit suffix is now authoritative (.HK, .SH/.SZ/.BJ, .US), then bare numeric codes classify by digit length (exactly 6 digits = A-share; ≤5 = HK). The old prefix heuristics misrouted bare 0700→SZ, 9988→US, 300750→HK, and even suffixed 830799.BJ→US. Prefix alone can't separate HK from A-share (both use leading 0/3); digit length can.
• _normalize_symbol (new) — canonicalizes bare tickers to the loader symbol form before fetching (AAPL→AAPL.US, 600000→600000.SH, 0700→0700.HK), while crypto pairs and already-suffixed codes pass through untouched. The user's original input is kept as the matrix label.
• Fetch loop walks the full FALLBACK_CHAINS until a loader actually returns data, instead of stopping at the first available one — a loader can be available yet serve nothing (network error), which silently dropped assets a later loader could serve. Every per-source failure is now logged (logger.warning) instead of swallowed by bare except: continue, so the next person debugging this sees why.

Tests (agent/tests/test_correlation.py, +10 cases): bare/suffixed classification for all markets incl. BJ, symbol normalization, and a chain fall-through regression test (first loader available-but-empty → second loader serves). 22 pass.

Notes

• 000001-style true ambiguity (Ping An Bank vs SSE Composite index) can't be resolved by inference — bare 6-digit codes default to the stock reading; the index still needs an explicit 000001.SH.
• Behavior for already-suffixed symbols and crypto pairs is unchanged.
• Touches only correlation.py + its tests; verified end-to-end: AAPL,SPY → 2×2 matrix (corr 0.4057), 0700,9988,AAPL,SPY → full 4×4 with eastmoney down (falls through to yahoo, failure logged).

Closes #471.

🤖 Generated with Claude Code

[warren618]

Merged — thanks for the root-cause work. The digit-length classification is the right disambiguator, and the available-but-empty chain fall-through closes a real silent-drop bug. Verified locally: 22 correlation tests green, AAPL,SPY end-to-end.