Docs: unify TraceLens metrics reference (CSV reports + TraceIndex SQL)

[ajassani]

Context

Follow-up from Kyle's doc review on TraceIndex PR #674. Doc changes were split into a separate PR so #674 stays code-focused.

Scope

• Document --report-root\ in the TraceIndex how-to
• Replace \perf-report-columns.md\ with \ racelens-metrics.md: shared intro, CSV/Excel section, and TraceIndex SQLite catalog section
• Update TOC and cross-links across how-to and conceptual docs

Implementation

PR: #981 (stacked on #674)

Acceptance

• Read the Docs / sphinx preview renders \ racelens-metrics\ and updated \ race-index\ how-to
• Cross-links from conceptual and how-to pages resolve correctly
• Kyle's doc comments on TraceIndex: build queryable TraceLens report catalogs #674 addressed