Add privacy engine, landing experience and dashboard shell

The strategy engine is pure and deterministic: no Math.random, no ambient
clock, seed and `now` passed in. That is what makes a plan reproducible
and lets the hard constraints be tested rather than asserted — 80 tests
cover the never-unshield rule, the 48h amount-reuse ban, privacy-floor
refusal, round-number avoidance and byte-identical determinism.

generatePlan returns a PlanResult rather than a bare array so refused
actions surface with their reason instead of vanishing, which matters
when the refusal is the product behaving correctly.

Landing page is scroll-scrubbed rather than fade-on-entry: the lifecycle
diagram draws the identity link re-forming at the reader's pace, and the
attacker view accumulates both ledgers side by side. Both are slices of
the real product surface, not marketing mockups.

Hero display type is capped at 6.75rem so the primary call to action
clears the fold on a 900px laptop.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>