feat(content): make the reach-saturation curve interactive (foundations-08)

[scumunna]

Roadmap depth ("make it do, not read"), applied to the one clean static-chart conversion a full audit surfaced.

Change

foundations-08 illustrated reach saturation R = Rmax(1 − e^(−kx)) with a static lineChart that was the section's only visual. Swapped it for the live responseCurve — the learner now drags spend and watches each extra dollar buy less new reach (the diminishing returns the lesson teaches, made manipulable). The self-contained predict after it is unchanged. ./Scripts/test.sh → 5,271 pass.

Audit finding (why just one chart)

I audited all 164 static charts. The headline count overstates the opportunity:

• Role tracks are already mature. Most static charts are intentional lead-ins to an interactive the lesson already has later (planning-07 → budgetAllocator, dsp-07 → pacingController, dsp-03/foundations-05), load-bearing worked-example data the prose narrates (foundations-11's spend ladder), or pure-data benchmarks (CPM-by-format, ad-tech-tax donut) that a generic widget can't represent. Converting those would duplicate widgets or break the lesson.
• foundations-08 was the lone clean win — a generic saturation curve with no specific labeled data to lose.
• Verticals hold ~33 modeled-relationship charts (funnels, pacing/flighting, reach), but they carry vertical-specific labeled stages (e.g., "VDP views → leads → test-drives → titled sales") that a generic funnelExplorer can't reproduce — so most would be a specificity downgrade, not an upgrade.

Recommendation: the static-chart lever is largely exhausted with this PR. Genuine further interactivity would mean new parameterized widgets (e.g., a funnel that accepts custom stages), which is engine work, not chart-swapping.

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