feat(runtime): add URL deep-linking and hashchange navigation

[Reebz]

What

Reload always landed on slide one, a link could not target a specific slide, and a deep-linked chart slide rendered blank because the chart and notes bootstrap hardcoded slide zero.

• slideFromHash() parses a one-based #N, clamps it to the valid range, and defaults to slide one. current initializes from it, goTo() writes history.replaceState(null, '', '#' + (current + 1)), and a hashchange listener handles Back and manual hash edits. The existing index === current guard prevents a feedback loop.
• The chart bootstrap and the load-time speaker-notes call now read from current instead of a hardcoded slide zero, so a deep-linked chart slide draws and the right notes log.

Mirrored into assets/slides-runtime.js and the inline runtime in references/html-template.md, and documented in references/presentation-layer.md.

Test

• Loading #5 opens slide five with its charts rendered.
• Reloading on slide seven returns to slide seven.
• Navigating updates the hash, and Back returns to the previous slide.
• An out-of-range or malformed hash clamps to a valid slide, and a deck with no hash starts on slide one as before.

Dependencies

Touches slides-runtime.js, so it is cleanest to merge after the accessibility PR.

Co-Authored-By: Leslie Barbara Knope (Claude Opus 4.8 (1M context)) noreply@anthropic.com

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