Draft: Add filtered token usage views

[vincepr]

Summary

Explores token accounting as alternate views over the sessions already discovered and filtered by sfind. This is a preserved draft, not merge-ready work.

Changes

• Adds normalized per-session, per-model token usage for Codex, OpenCode, and Claude Code.
• Reads OpenCode recorded costs while leaving unavailable Codex and Claude costs unknown.
• Adds an internal cost-provider boundary for later pricing implementations.
• Cycles Tab through Continue, Fork, Usage, and Graph modes.
• Aggregates the Usage view over sessions matching the current text, date, and provider filters.
• Adds a graph approximation that assigns each session lifetime usage to its last-activity timestamp.
• Splits selected date ranges into width-aware minute, hour, day, and longer intervals.
• Stacks model usage with stable colors and groups the long tail into Other.
• Adds provider parsing, aggregation, mode, interval, and model-grouping regression tests.

Verification

• cargo test passed after the latest graph implementation.
• git diff --check main...HEAD passes.
• The full required fmt check, Clippy, and documentation suite was run before the final graph refactor, but has not been rerun against this exact snapshot.

Risks/Notes

Remaining work before this should be considered mergeable:

• Run cargo fmt -- --check, cargo test, cargo clippy --all-targets --all-features -- -D warnings, and cargo doc --no-deps against the final snapshot.
• Update the README wording for sub-day intervals, stacked model colors, and Other grouping.
• Manually validate the TUI with real Codex, OpenCode, and Claude data on narrow and wide terminals.
• Review stacked-bar rounding, legends on low-height terminals, interval labels, and empty buckets.
• Reassess whether the graph approximation is useful enough: it assigns full session totals to last activity and is not true event-level history.
• Review Codex fork/replay behavior because copied usage history can inflate per-session aggregate totals.
• Decide whether to keep this implementation or use it only as reference for a smaller redesign.