chore(release): prepare v2.6.0

[mmcky]

Release prep for v2.6.0, per the flow in CONTRIBUTING.md. Bundles the three parity phases that landed since v2.5.0.

Included

Change	Phase	PR
QuantEcon code-token palette (highlight.js mapping of qetheme_code_style) and seoul256 text colours for emphasis, strong text and definition terms	3 (#89)	#171
Language switcher and hreflang alternates from the languages / current_language site options	4 (#90)	#174
Right-to-left editions via enable_rtl, with current_language setting the document lang	5 (#91)	#174
Translator credit in the page header from translators / translators_label, with a per-page override	#143	#174
The six new site options declared in template.yml (lists carried as YAML block strings)		#174

#175 (a comment correction) needs no entry.

What this PR changes

• CHANGELOG.md: the Unreleased entries move under ## [2.6.0] - 2026-09-10, the compare link is added, and the Unreleased link now compares from v2.6.0.
• package.json / package-lock.json: version 2.6.0. template.yml is stamped by the release workflow.
• PLAN.md: Phases 3, 4 and 5 marked as shipped in v2.6.0.

After merge

Tag the merge commit and push the tag; release.yml builds the bundle and publishes the GitHub Release with this changelog section as the notes:

git tag v2.6.0 <merge-sha> && git push origin v2.6.0

Consumers pin releases explicitly, so nothing moves until a lecture repo bumps its site.template URL.

🤖 Generated with Claude Code

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-09-10 07:21 UTC

[Copilot]

🟢 Approval recommended

The changes are a mechanical, low-risk release bump that exactly follows the documented CONTRIBUTING.md flow, with version numbers, changelog structure, compare links, and date all verified consistent.

Pull request overview

This PR prepares the v2.6.0 release, following the manual release flow documented in CONTRIBUTING.md. It bundles three already-merged parity phases (Phase 3 code/text palette via #171, and Phases 4–5 i18n/RTL plus the translator credit via #174) into a single tagged release. No product code changes — this is purely release bookkeeping.

Changes:

• Move the ## [Unreleased] CHANGELOG entries under a new ## [2.6.0] - 2026-09-10 heading, add the [2.6.0] compare link, and re-point [Unreleased] to v2.6.0...HEAD.
• Bump the version to 2.6.0 in package.json and package-lock.json (both fields).
• Update PLAN.md to mark Phases 3, 4 and 5 as shipped in v2.6.0.

File summaries

File	Description
CHANGELOG.md	Moves Unreleased entries under ## [2.6.0] - 2026-09-10; adds/updates compare links, matching the documented flow and the release.yml extraction format.
package.json	Version bump 2.5.0 → 2.6.0, the source of truth the release workflow stamps into the bundle.
package-lock.json	Mirrors the version bump in both the top-level and root-package fields.
PLAN.md	Marks Phases 3–5 as shipped in v2.6.0 and updates the shipped-state narrative.

I verified: version consistency across package.json/package-lock.json; the CHANGELOG heading format matches the awk guard in release.yml (## [2.6.0]); the compare links are correct; the release date 2026-09-10 equals today (not a future date); template.yml is correctly left unbumped since release.yml stamps it from package.json; and remaining 2.5.0 references are unrelated dependency versions or historical changelog links.

Review details

• Files reviewed: 3/4 changed files
• Comments generated: 0
• Review effort level: Balanced

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[github-actions]

🎭 Visual regression results

 31 passed
 5 skipped

Details

 36 tests across 1 suite
 59.1 seconds
 10f8fab

Skipped tests

mobile-chrome › theme.spec.ts › QuantEcon theme — visual regression › without JavaScript › drawer-opens-without-javascript
mobile-chrome › theme.spec.ts › QuantEcon theme — visual regression › drawer-closes-when-search-opens
mobile-chrome › theme.spec.ts › QuantEcon theme — visual regression › launch-colab
mobile-chrome › theme.spec.ts › QuantEcon theme — visual regression › live-compute-toggle
mobile-chrome › theme.spec.ts › QuantEcon theme — visual regression › live-compute-toggle-absent-without-thebe