Align HTML usage activity dates with their cells

Traceability

• Spec ID: 2026-08-01-html-usage-activity-date-alignment
• Status: Implemented
• Issue: None; focused report-layout maintenance from visual review of the generated AI coding workflow evidence report.

Intent

Make the self-contained HTML report project-usage activity chart preserve the one-to-one relationship between each UTC date and its activity cell. Dates must read from left to right, use the same grid columns as the cells they describe, and keep any overflow inside the chart instead of expanding the page.

Acceptance Scenarios

• HUA-AC-1: The HTML renderer emits exactly one activity cell per retained UTC date in source order, with strictly increasing horizontal grid columns and the existing heat level, tooltip, and accessible label derived from that date's value.
• HUA-AC-2: The activity axis shares the chart's --heat-days, column width, and gap. Ranges of four days or fewer label every date in its matching column. Longer ranges label the first and last dates plus every seventh date, excluding periodic ticks within the last three days to avoid a duplicate end cluster.
• HUA-AC-3: At 1180 px, 800 px, and 320 px viewport widths, the activity panel remains vertically centered, the page has no horizontal overflow, and long date ranges scroll only inside the chart container.
• HUA-AC-4: Empty activity data, English and Chinese output, tooltip and aria-label content, self-contained HTML behavior, artifact names, and the summary.usageActivity schema, UTC ordering, values, scoring, findings, Markdown, and Canvas behavior remain unchanged.
• HUA-AC-5: The existing findings in .codex/better-harness/2026-08-01-ai-coding-workflow-evidence re-render to the same canonical artifact set, and renderer plus HTML validation pass.
• HUA-AC-6: Focused renderer tests, the documentation graph and link checks, the full package test suite, package verification, preview health and Canvas runtime smoke checks, browser console/page-error inspection, and whitespace validation are executed and recorded. Feature-related checks pass; unavailable external runtime or platform privileges remain explicit exceptions.

Non-goals

• Changing activity collection, date normalization, truncation, aggregation, heat calculation, scoring, finding eligibility, or report conclusions.
• Adding report fields, JavaScript, external resources, or generated artifacts.
• Changing the Markdown or Canvas renderer, the activity schema, or the generated report's evidence source.
• Hand-editing the generated report.html or changing unrelated root CLI, test, spec, documentation-graph, or diagnostic-output worktree changes.

Plan and Tasks

1. Add renderer-level fixtures for four-day, thirty-day, empty, and localized activity output, with explicit date-to-column assertions. (HUA-AC-1, HUA-AC-2, HUA-AC-4)
2. Replace the seven-row column flow with one shared horizontal cell-and-axis grid, deterministic tick selection, and chart-local overflow. (HUA-AC-1, HUA-AC-2, HUA-AC-3)
3. Document the portable HTML activity-axis contract without changing report data ownership or runtime boundaries. (HUA-AC-2, HUA-AC-4)
4. Re-render the current report from its existing findings.json, then run automated, visual, preview, packaging, and traceability checks before changing this spec to Implemented. (HUA-AC-3, HUA-AC-5, HUA-AC-6)

Test and Review Evidence

• The focused activity-chart regressions passed 3/3: node --test --test-name-pattern="HTML activity chart" test/harness-report-render-cli.test.mjs. They cover four-day column binding, thirty-day tick selection, empty data, Chinese output, tooltip/aria-label parity, and self-contained HTML.
• Re-rendering the current report from its existing findings.json passed all four canonical checks (output-location, run-directory-artifacts, findings-json, and html-report). The pre-existing fix-result sidecar was moved outside the run directory only for validation and restored unchanged. SHA-256 checks confirmed findings.json, report.md, and the sidecar were unchanged; only report.html changed.
• Playwright geometry checks at 1180 px, 800 px, and 320 px found four strictly increasing cells, matching source-date/grid-column pairs, zero pixel cell-to-label center deltas, and zero page-level horizontal overflow. A thirty-day browser sequence at 320 px produced 256 px of chart-local overflow with overflow-x:auto; the page still had zero horizontal overflow. The final browser console check reported zero errors and zero warnings.
• Documentation routing regenerated to 35 files and 51 links, and the doc-link suite passed 6/6: node scripts/doc-link-graph/cli.mjs skills/better-harness and node --test test/doc-link-graph.test.mjs.
• Package verification passed with 369 npm entries and 392 runtime-zip entries: npm run pack:verify.
• The full suite completed with 1044 passes, 6 platform skips, and 4 failures: npm test. Every failure was an existing Windows fixture that could not create a symbolic link because the current process lacks symlink authority (EPERM); no HTML or activity-chart assertion failed. The direct renderer suite likewise passed 29/30 with only its symlink fixture blocked.
• npm run preview was executed but intentionally failed fast because this machine has no Canvas SDK runtime and neither CANVAS_SDK_MEDIA_DIR nor CANVAS_SDK_ROOT is configured. Consequently /health and /canvas-module.js could not be smoke-tested in this environment; no fake runtime was substituted for that missing external prerequisite.