v0.7.7: make spend/cost display monotonic and source-backed

[Hmbown]

Problem

In live use, the displayed spend/cost appeared to go down during or after sub-agent/fanout activity. Even if token accounting is approximate, the user-visible total should not decrease unless there is an explicit reset or session switch.

This may be related to overlapping parent/sub-agent usage accounting, prompt-cache adjustments, or replacing provisional estimates with lower final values, but the UI needs to make the source of truth clear.

Acceptance

• Session spend display is monotonic within a session unless a clearly labelled reset/session change occurs.
• Parent and sub-agent usage events are deduplicated without subtracting already-displayed totals in a confusing way.
• If provisional estimates are replaced by final provider usage, the UI communicates that or tracks separate estimated/final fields.
• Cache-hit/cached-token accounting does not make the visible total appear to move backwards.
• Add focused tests for out-of-order usage events, duplicate sub-agent events, and final-usage reconciliation.

Notes

Related to the broader telemetry/audit work in #204, but this issue tracks the concrete user-visible regression: spend should not visibly decrease during a live run.

[Hmbown]

v0.7.7 stabilization: implemented and test-verified.

PR #246 (codex/v0.7.7-subagent-stabilization).

What changed

• New App::displayed_cost_high_water: f64 anchors the visible total. Both render sites (render_footer_from, footer_status_line_spans) call App::displayed_session_cost() which returns max(session_cost + subagent_cost, displayed_cost_high_water).
• All cost mutations route through accrue_session_cost(delta) / accrue_subagent_cost(delta), which advance the high-water mark. Direct += is no longer used at the call sites.
• Existing mailbox dedup (subagent_cost_event_seqs) is preserved; duplicate events are skipped at the dedup layer, so the displayed cost is doubly safe.
• Net effect: cache reconciliation, provisional → final swaps, or out-of-order events can lower the underlying counters, but the displayed value never decreases within a session.

Tests

• displayed_session_cost_is_monotonic_under_negative_reconciliation — accrues 0.50, then writes a smaller value to subagent_cost, asserts displayed cost did not regress
• duplicate_mailbox_token_usage_does_not_regress_displayed_cost — same seq dedups; fresh seq advances

Install proof: deepseek 0.7.7 on /Users/hunterbown/.cargo/bin/deepseek.

[Hmbown]

Closing: this shipped in v0.7.7 and remains present on current main/release history. The implementation, focused test coverage, and local install proof are already documented on this issue in the PR #246 verification comment.

[4pito3pito2pi]

deepseek can litterally curl the api and request bala ce. balance old minus blance new is spend.