perf: skip unused callers JSON decode in ProfileStats

[KRRT7]

Summary

• Skip the callers BLOB column in ProfileStats.create_stats() — it was fetched and JSON-decoded for every row but never used by FunctionRanker
• Eliminates O(n) JSON parsing at ranking startup (the dominant cost in FunctionRanker.__init__)
• Fixes all 20 pre-existing mypy errors in profile_stats.py by declaring pstats.Stats dynamic attributes

Context

Part of #2132 (E2E performance, item 9). The callers column is written by the tracer but never read back — FunctionRanker.load_function_stats() unpacks it as _callers and discards it, and call_graph.py uses its own query that already excludes the column.

Stack

1. perf: prefilter files by path before read_text() in discovery #2134 (prefilter) → main
2. perf: cache file reads and AST parses during discovery pass #2135 (ast-cache) → perf/discovery-prefilter
3. perf: read JS/TS files once during discovery export checks #2136 (js-read) → perf/discovery-ast-cache
4. This PR → perf/discovery-js-read

Test plan

• All 6 tests/test_function_ranker.py tests pass
• mypy clean (0 errors, down from 20 pre-existing)
• ruff check + format pass