feat: canonical proposal and managed-service policy

[yashodhank]

Outcome

Introduces the canonical Founder Managed policy, separates provider tax from Securiace output GST, unifies FX and owner-margin math across report surfaces, and adds a client-safe proposal workspace with deterministic exports.

Key safeguards

• client projections are allow-listed; internal/silent facts never enter client HTML/JSON/CSV/copy
• managed tiers, monthly expiry, discretionary non-contractual credits, ₹2,500/hour overage, and emergency boundaries are versioned
• output GST defaults off and fails closed without verified registration/commercial mode
• 45% displayed markup remains 45% in every surface
• Codex generation is capability-detected and remains disabled on the current server
• current legacy taxonomy is visibly marked not release-current
• Plan 005 consolidates Plans 001–004 and records production dark-launch/migration gates

Validation

• 34 Node contract tests
• cargo fmt + clippy -D warnings
• 7 Rust unit, 19 API smoke, 4 schema contract tests
• direct release-binary Playwright audit at 1280x1000: managed quantity, internal-note privacy, same-origin FX, sticky actions, zero browser errors

Supersession

This is the clean successor from current main. PRs #24 and #25 remain untouched until this successor and its dependent slices are reviewed.

Not in this PR

• Rust CLI flag/current taxonomy repair (separate sibling branch)
• WHMCS Proposal Studio source port (dependent PR)
• production deployment or provider writes