fix(reports): align usage dates with activity cells

[sxyseo]

Summary

• render retained UTC activity dates as one horizontal daily grid
• bind activity cells and date ticks to the same columns, with sparse long-range ticks and chart-local scrolling
• preserve heat levels, tooltips, accessibility labels, report data, Markdown, and Canvas behavior
• document the HTML contract and add short-range, long-range, empty, localized, and self-contained regression coverage

Validation

• node --test --test-name-pattern="HTML activity chart" test/harness-report-render-cli.test.mjs (3/3 passed)
• canonical current-report render validation (output location, artifact set, findings JSON, and HTML report all passed)
• Playwright at 1180 px, 800 px, and 320 px (matching columns, zero short-range center delta, no page overflow, console clean)
• node --test test/doc-link-graph.test.mjs (6/6 passed)
• npm run pack:verify (npm 369 entries, runtime zip 392 entries)
• npm test (1044 passed, 6 skipped; 4 Windows symlink fixtures blocked by local EPERM)

Environment note

npm run preview could not start locally because no Canvas SDK runtime is installed or configured. No fake runtime was substituted; CI can exercise the supported runtime path.

[Copilot]

Pull request overview

This PR updates the HTML report renderer so usage-activity dates (tick labels) are horizontally aligned with their corresponding activity cells using a shared grid-column system, while keeping the report self-contained and preserving existing activity semantics (heat levels, tooltips, aria-labels, and data schema). It also adds renderer-level regression tests and documents the HTML contract expectations for the activity chart.

Changes:

• Update HTML activity chart rendering to bind cells and ticks to the same explicit grid-column positions and move overflow scrolling inside the chart container.
• Add deterministic long-range tick selection (activityTickIndexes) for sparse labeling while preserving short-range per-day labeling.
• Add focused test coverage for short-range, long-range, empty, localized, accessibility, and self-contained HTML output, plus spec/docs updates describing the contract.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/harness-report-render-cli.test.mjs	Adds targeted regression tests asserting cell/tick column binding, sparse tick selection, empty/localized output, and self-contained HTML constraints.
templates/reporting/html-visual.md	Documents the HTML contract for left-to-right UTC date rendering and shared grid-column alignment with chart-local scrolling.
scripts/harness-analysis/renderers/html.mjs	Implements shared column-based layout (data-date, grid-column styles), sparse tick logic, and chart-local scroll container/CSS.
docs/specs/2026-08-01-html-usage-activity-date-alignment.md	Adds a spec with acceptance scenarios and recorded test/validation evidence for the activity-date alignment change.

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